We've successfully reproduced the error on a newly built instance.  We'll proceed to run the mainline kernel on the same instance.

Worth noting that it took 441529 seconds (5.1 days) since boot for this to manifest in this case.  On our previous instance the time to failure was in the order of a hours to days.

[441529.300238] ------------[ cut here ]------------
[441529.300288] WARNING: CPU: 1 PID: 10409 at /build/linux-lts-xenial-N0BD4p/linux-lts-xenial-4.4.0/net/ipv6/addrconf_core.c:159 in6_dev_finish_destroy+0x6b/0xb0()
[441529.300359] CPU: 1 PID: 10409 Comm: kworker/u4:0 Not tainted 4.4.0-87-generic #110~14.04.1-Ubuntu
[441529.300361] Hardware name: VMware, Inc. VMware Virtual Platform/440BX Desktop Reference Platform, BIOS 6.00 09/17/2015
[441529.300374] Workqueue: netns cleanup_net
[441529.300376]  0000000000000000 ffff880088ddbbb8 ffffffff813de2dc 0000000000000000
[441529.300378]  ffffffff81d7aa60 ffff880088ddbbf0 ffffffff8107ea46 ffff880139e5a400
[441529.300380]  0000000000000000 0000000000000006 ffff880088ddbcc0 0000000000000000
[441529.300382] Call Trace:
[441529.300398]  [<ffffffff813de2dc>] dump_stack+0x63/0x87
[441529.300413]  [<ffffffff8107ea46>] warn_slowpath_common+0x86/0xc0
[441529.300415]  [<ffffffff8107eb3a>] warn_slowpath_null+0x1a/0x20
[441529.300417]  [<ffffffff817e8b9b>] in6_dev_finish_destroy+0x6b/0xb0
[441529.300421]  [<ffffffff817be360>] ip6_route_dev_notify+0x110/0x130
[441529.300425]  [<ffffffff8109dde9>] notifier_call_chain+0x49/0x70
[441529.300427]  [<ffffffff8109df06>] raw_notifier_call_chain+0x16/0x20
[441529.300429]  [<ffffffff81701d35>] call_netdevice_notifiers_info+0x35/0x60
[441529.300432]  [<ffffffff8170c007>] netdev_run_todo+0x157/0x300
[441529.300436]  [<ffffffff81703a4e>] ? rollback_registered_many+0x22e/0x2e0
[441529.300440]  [<ffffffff817172fe>] rtnl_unlock+0xe/0x10
[441529.300442]  [<ffffffff81704d98>] default_device_exit_batch+0x138/0x150
[441529.300452]  [<ffffffff810bf540>] ? __wake_up_sync+0x20/0x20
[441529.300454]  [<ffffffff816fd252>] ops_exit_list.isra.4+0x52/0x60
[441529.300456]  [<ffffffff816fe223>] cleanup_net+0x1b3/0x280
[441529.300461]  [<ffffffff81096d90>] process_one_work+0x150/0x3f0
[441529.300463]  [<ffffffff8109750a>] worker_thread+0x11a/0x470
[441529.300471]  [<ffffffff81808d99>] ? __schedule+0x359/0x980
[441529.300473]  [<ffffffff810973f0>] ? rescuer_thread+0x310/0x310
[441529.300475]  [<ffffffff8109cdc6>] kthread+0xd6/0xf0
[441529.300477]  [<ffffffff81081875>] ? do_exit+0x655/0xae0
[441529.300479]  [<ffffffff8109ccf0>] ? kthread_park+0x60/0x60
[441529.300481]  [<ffffffff8180d00f>] ret_from_fork+0x3f/0x70
[441529.300483]  [<ffffffff8109ccf0>] ? kthread_park+0x60/0x60
[441529.300484] ---[ end trace 5e09761f8674a0af ]---
[441529.300485] ------------[ cut here ]------------
[441529.300487] WARNING: CPU: 1 PID: 10409 at /build/linux-lts-xenial-N0BD4p/linux-lts-xenial-4.4.0/net/ipv6/addrconf_core.c:160 in6_dev_finish_destroy+0xa4/0xb0()
[441529.300510] CPU: 1 PID: 10409 Comm: kworker/u4:0 Tainted: G        W       4.4.0-87-generic #110~14.04.1-Ubuntu
[441529.300511] Hardware name: VMware, Inc. VMware Virtual Platform/440BX Desktop Reference Platform, BIOS 6.00 09/17/2015
[441529.300513] Workqueue: netns cleanup_net
[441529.300514]  0000000000000000 ffff880088ddbbb8 ffffffff813de2dc 0000000000000000
[441529.300515]  ffffffff81d7aa60 ffff880088ddbbf0 ffffffff8107ea46 ffff880139e5a400
[441529.300517]  0000000000000000 0000000000000006 ffff880088ddbcc0 0000000000000000
[441529.300518] Call Trace:
[441529.300520]  [<ffffffff813de2dc>] dump_stack+0x63/0x87
[441529.300522]  [<ffffffff8107ea46>] warn_slowpath_common+0x86/0xc0
[441529.300524]  [<ffffffff8107eb3a>] warn_slowpath_null+0x1a/0x20
[441529.300526]  [<ffffffff817e8bd4>] in6_dev_finish_destroy+0xa4/0xb0
[441529.300528]  [<ffffffff817be360>] ip6_route_dev_notify+0x110/0x130
[441529.300529]  [<ffffffff8109dde9>] notifier_call_chain+0x49/0x70
[441529.300531]  [<ffffffff8109df06>] raw_notifier_call_chain+0x16/0x20
[441529.300533]  [<ffffffff81701d35>] call_netdevice_notifiers_info+0x35/0x60
[441529.300535]  [<ffffffff8170c007>] netdev_run_todo+0x157/0x300
[441529.300538]  [<ffffffff81703a4e>] ? rollback_registered_many+0x22e/0x2e0
[441529.300540]  [<ffffffff817172fe>] rtnl_unlock+0xe/0x10
[441529.300543]  [<ffffffff81704d98>] default_device_exit_batch+0x138/0x150
[441529.300546]  [<ffffffff810bf540>] ? __wake_up_sync+0x20/0x20
[441529.300549]  [<ffffffff816fd252>] ops_exit_list.isra.4+0x52/0x60
[441529.300552]  [<ffffffff816fe223>] cleanup_net+0x1b3/0x280
[441529.300555]  [<ffffffff81096d90>] process_one_work+0x150/0x3f0
[441529.300558]  [<ffffffff8109750a>] worker_thread+0x11a/0x470
[441529.300561]  [<ffffffff81808d99>] ? __schedule+0x359/0x980
[441529.300563]  [<ffffffff810973f0>] ? rescuer_thread+0x310/0x310
[441529.300564]  [<ffffffff8109cdc6>] kthread+0xd6/0xf0
[441529.300566]  [<ffffffff81081875>] ? do_exit+0x655/0xae0
[441529.300568]  [<ffffffff8109ccf0>] ? kthread_park+0x60/0x60
[441529.300570]  [<ffffffff8180d00f>] ret_from_fork+0x3f/0x70
[441529.300571]  [<ffffffff8109ccf0>] ? kthread_park+0x60/0x60
[441529.300573] ---[ end trace 5e09761f8674a0b0 ]---
[441529.300775] PGD b97cd067 PUD b97ad067 PMD 0 
[441529.300877] Oops: 0000 [#1] SMP 
[441529.301242] CPU: 1 PID: 10409 Comm: kworker/u4:0 Tainted: G        W       4.4.0-87-generic #110~14.04.1-Ubuntu
[441529.301281] Hardware name: VMware, Inc. VMware Virtual Platform/440BX Desktop Reference Platform, BIOS 6.00 09/17/2015
[441529.302097] Workqueue: netns cleanup_net
[441529.302114] task: ffff880057de0e00 ti: ffff880088dd8000 task.ti: ffff880088dd8000
[441529.302143] RIP: 0010:[<ffffffff817e8b5d>]  [<ffffffff817e8b5d>] in6_dev_finish_destroy+0x2d/0xb0
[441529.328032] RSP: 0018:ffff880088ddbc10  EFLAGS: 00010246
[441529.328064] RAX: 0000000000000024 RBX: ffff880139e5a400 RCX: 0000000000000006
[441529.328092] RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000009
[441529.328120] RBP: ffff880088ddbc20 R08: 000000002d2d2d5d R09: 3520656361727420
[441529.328147] R10: 3866313637393065 R11: 000000000003183f R12: 0000000000000000
[441529.328175] R13: 0000000000000006 R14: ffff880088ddbcc0 R15: 0000000000000000
[441529.328206] FS:  0000000000000000(0000) GS:ffff88013fd00000(0000) knlGS:0000000000000000
[441529.328237] CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
[441529.328260] CR2: 0000000000000448 CR3: 00000000b9743000 CR4: 00000000000406e0
[441529.328351] Stack:
[441529.328363]  ffff8800b0349880 0000000000000000 ffff880088ddbc38 ffffffff817be360
[441529.328397]  00000000ffffffed ffff880088ddbc70 ffffffff8109dde9 ffff880088ddbcc0
[441529.328429]  0000000000000006 ffff88002397d000 0000000106932e35 00000000000000fa
[441529.329355] Call Trace:
[441529.330187]  [<ffffffff817be360>] ip6_route_dev_notify+0x110/0x130
[441529.331043]  [<ffffffff8109dde9>] notifier_call_chain+0x49/0x70
[441529.331927]  [<ffffffff8109df06>] raw_notifier_call_chain+0x16/0x20
[441529.332740]  [<ffffffff81701d35>] call_netdevice_notifiers_info+0x35/0x60
[441529.333546]  [<ffffffff8170c007>] netdev_run_todo+0x157/0x300
[441529.334360]  [<ffffffff81703a4e>] ? rollback_registered_many+0x22e/0x2e0
[441529.335187]  [<ffffffff817172fe>] rtnl_unlock+0xe/0x10
[441529.335981]  [<ffffffff81704d98>] default_device_exit_batch+0x138/0x150
[441529.336771]  [<ffffffff810bf540>] ? __wake_up_sync+0x20/0x20
[441529.337534]  [<ffffffff816fd252>] ops_exit_list.isra.4+0x52/0x60
[441529.338296]  [<ffffffff816fe223>] cleanup_net+0x1b3/0x280
[441529.339013]  [<ffffffff81096d90>] process_one_work+0x150/0x3f0
[441529.339750]  [<ffffffff8109750a>] worker_thread+0x11a/0x470
[441529.340448]  [<ffffffff81808d99>] ? __schedule+0x359/0x980
[441529.341116]  [<ffffffff810973f0>] ? rescuer_thread+0x310/0x310
[441529.341820]  [<ffffffff8109cdc6>] kthread+0xd6/0xf0
[441529.342471]  [<ffffffff81081875>] ? do_exit+0x655/0xae0
[441529.343102]  [<ffffffff8109ccf0>] ? kthread_park+0x60/0x60
[441529.343696]  [<ffffffff8180d00f>] ret_from_fork+0x3f/0x70
[441529.344306]  [<ffffffff8109ccf0>] ? kthread_park+0x60/0x60
[441529.344890] Code: 66 66 90 55 48 8d 47 08 48 89 e5 41 54 53 48 39 47 08 48 89 fb 4c 8b 27 75 3e 48 83 7b 18 00 75 70 48 83 bb b8 02 00 00 00 75 53 <49> 8b 84 24 48 04 00 00 65 ff 08 8b 83 58 01 00 00 85 c0 74 2b 
[441529.347349]  RSP <ffff880088ddbc10>
[441529.347956] CR2: 0000000000000448
[441529.349682] ---[ end trace 5e09761f8674a0b1 ]---