linux 4.4.0-142.168 ADT test failure (ubuntu_vfat_stress): hung task on i386

Testing failed on:
    i386: https://objectstorage.prodstack4-5.canonical.com/v1/AUTH_77e2ada1e7a84929a74ba3b87153c0ac/autopkgtest-xenial/xenial/i386/l/linux/20190201_082722_4cb3e@/log.gz

ubuntu_vfat_stress is causing a kernel hung task on xenial/linux i386:

Started, PID 23212
  Timed out waiting for stress-ng to finish
  Forcefully killing, PID 23212 (after 121 seconds)
  Found kernel warning and/or call trace:

  [ 3998.807995] Testing: --verify --times --metrics-brief --syslog --keep-name -t 10s --hdd 2 --hdd-opts sync,wr-rnd,rd-rnd,fadv-willneed,fadv-rnd --lockf 2 --seek 2 --aio 2 --aio-requests 32 --dentry 2 --dir 2 --dentry-order stride --fallocate 2 --fstat 2 --dentries 100 --lease 2 --open 2 --rename 2 --hdd-bytes 4M --fallocate-bytes 2M --chdir 2 --rename 2 --hdd-write-size 512
  [ 3998.832802] Created loop image vfat-test-23176/vfat-image-23176.img on /dev/loop0
  [ 3999.936713] ubuntu_vfat_str (23176): drop_caches: 1
  [ 3999.944908] ubuntu_vfat_str (23176): drop_caches: 2
  [ 3999.949450] ubuntu_vfat_str (23176): drop_caches: 3
  [ 3999.967248] VFAT options: allow_utime=20
  [ 3999.967299] Stress test: /tmp/autopkgtest.ytglRN/build.6Ql/src/autotest/client/tmp/ubuntu_vfat_stress/src/stress-ng/stress-ng --verify --times --metrics-brief --syslog --keep-name -t 10s --hdd 2 --hdd-opts sync,wr-rnd,rd-rnd,fadv-willneed,fadv-rnd --lockf 2 --seek 2 --aio 2 --aio-requests 32 --dentry 2 --dir 2 --dentry-order stride --fallocate 2 --fstat 2 --dentries 100 --lease 2 --open 2 --rename 2 --hdd-bytes 4M --fallocate-bytes 2M --chdir 2 --rename 2 --hdd-write-size 512
  [ 3999.967318] Mount point: /mnt/vfat-test-23176
  [ 3999.967994] Started, PID 23212
  [ 4121.902337] Timed out waiting for stress-ng to finish
  [ 4121.902488] Forcefully killing, PID 23212 (after 121 seconds)
  [ 4200.180102] INFO: task stress-ng:23229 blocked for more than 120 seconds.
  [ 4200.184444] Tainted: G OE 4.4.0-142-generic #168-Ubuntu
  [ 4200.185438] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
  [ 4200.186581] stress-ng D c1214654 0 23229 1 0x00000004
  [ 4200.186587] c5bc9ebc 00000086 c5bc9ea4 c1214654 00000000 ce0a5b00 64d56897 000003a3
  [ 4200.186591] 6497d7cf 000003a3 d8185dc0 d239a800 c5bca000 d00ce5e0 c5bc9f08 c5bc9ec8
  [ 4200.186595] c17e194d c5bc9f08 c5bc9ef0 c12146ad 00000000 d239a800 c10b5340 d00ce5e4
  [ 4200.186598] Call Trace:
  [ 4200.186608] [<c1214654>] ? wb_queue_work+0xe4/0xf0
  [ 4200.186613] [<c17e194d>] schedule+0x2d/0x80
  [ 4200.186616] [<c12146ad>] wb_wait_for_completion+0x4d/0x90
  [ 4200.186620] [<c10b5340>] ? wake_atomic_t_function+0x70/0x70
  [ 4200.186622] [<c1217bdd>] sync_inodes_sb+0x7d/0x1a0
  [ 4200.186624] [<c121d625>] sync_inodes_one_sb+0x15/0x20
  [ 4200.186628] [<c11ef26d>] iterate_supers+0xcd/0xd0
  [ 4200.186630] [<c121d610>] ? SyS_tee+0x2a0/0x2a0
  [ 4200.186632] [<c121d95a>] sys_sync+0x3a/0xa0
  [ 4200.186636] [<c10038af>] do_fast_syscall_32+0x9f/0x160
  [ 4200.186639] [<c17e5bf0>] sysenter_past_esp+0x3d/0x61
  [ 4200.186641] INFO: task stress-ng:23242 blocked for more than 120 seconds.
  [ 4200.187628] Tainted: G OE 4.4.0-142-generic #168-Ubuntu
  [ 4200.188667] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
  [ 4200.189947] stress-ng D c1214654 0 23242 1 0x00000004
  [ 4200.189952] c011bebc 00000086 c011bea4 c1214654 00000000 dd922680 65d2d986 000003a3
  [ 4200.189955] 65bb3d70 000003a3 d8185dc0 cef0b200 c011c000 d00ce5e0 c011bf08 c011bec8
  [ 4200.189958] c17e194d c011bf08 c011bef0 c12146ad 00000000 cef0b200 c10b5340 c5bc9edc
  [ 4200.189962] Call Trace:
  [ 4200.189965] [<c1214654>] ? wb_queue_work+0xe4/0xf0
  [ 4200.189968] [<c17e194d>] schedule+0x2d/0x80
  [ 4200.189971] [<c12146ad>] wb_wait_for_completion+0x4d/0x90
  [ 4200.189973] [<c10b5340>] ? wake_atomic_t_function+0x70/0x70
  [ 4200.189975] [<c1217bdd>] sync_inodes_sb+0x7d/0x1a0
  [ 4200.189978] [<c121d625>] sync_inodes_one_sb+0x15/0x20
  [ 4200.189980] [<c11ef26d>] iterate_supers+0xcd/0xd0
  [ 4200.189983] [<c121d610>] ? SyS_tee+0x2a0/0x2a0
  [ 4200.189985] [<c121d95a>] sys_sync+0x3a/0xa0
  [ 4200.189987] [<c10038af>] do_fast_syscall_32+0x9f/0x160
  [ 4200.189989] [<c17e5bf0>] sysenter_past_esp+0x3d/0x61

This is not a regression, different call traces or errors have been reported by older versions.