refresh with layouts in both revisions fails

With one snap with layouts like this:

```
name: kubernetes-thing
version: "0.0.1"
summary: kubernetes things
description: things
base: core18
architectures:
  - amd64
confinement: strict
grade: devel
layout:
  /var/lib/kubelet:
    bind: $SNAP_DATA/kubelet
  /var/log:
    bind: $SNAP_DATA/log
apps:
  containerd:
    command: snap/command-chain/snapcraft-runner bin/bash -c sleep 1000
    daemon: simple
```

I can't refresh the snap if I have daemons or apps that ran and saved the mount ns, because snap-update-ns fails to run when refreshing. The specific error message with debug logs:

```
error: cannot perform the following tasks:
- Setup snap "kubernetes-thing" (unset) security profiles (cannot update mount namespace of snap "kubernetes-thing": cannot update preserved namespace of snap "kubernetes-thing":
-----
common.go:60: DEBUG: locking mount namespace of snap "kubernetes-thing"
common.go:81: DEBUG: freezing processes of snap "kubernetes-thing"
change.go:514: DEBUG: reusing synthetic entry "tmpfs /var/lib tmpfs x-snapd.synthetic,x-snapd.needed-by=/var/lib/kubelet,mode=0755,uid=0,gid=0 0 0"
change.go:514: DEBUG: reusing synthetic entry "/var/lib/cloud /var/lib/cloud none rbind,x-snapd.synthetic,x-snapd.needed-by=/var/lib/kubelet,x-snapd.detach 0 0"
change.go:514: DEBUG: reusing synthetic entry "/var/lib/console-conf /var/lib/console-conf none rbind,x-snapd.synthetic,x-snapd.needed-by=/var/lib/kubelet,x-snapd.detach 0 0"
change.go:514: DEBUG: reusing synthetic entry "/var/lib/dbus /var/lib/dbus none rbind,x-snapd.synthetic,x-snapd.needed-by=/var/lib/kubelet,x-snapd.detach 0 0"
change.go:514: DEBUG: reusing synthetic entry "/var/lib/dhcp /var/lib/dhcp none rbind,x-snapd.synthetic,x-snapd.needed-by=/var/lib/kubelet,x-snapd.detach 0 0"
change.go:514: DEBUG: reusing synthetic entry "/var/lib/extrausers /var/lib/extrausers none rbind,x-snapd.synthetic,x-snapd.needed-by=/var/lib/kubelet,x-snapd.detach 0 0"
change.go:514: DEBUG: reusing synthetic entry "/var/lib/misc /var/lib/misc none rbind,x-snapd.synthetic,x-snapd.needed-by=/var/lib/kubelet,x-snapd.detach 0 0"
change.go:514: DEBUG: reusing synthetic entry "/var/lib/pam /var/lib/pam none rbind,x-snapd.synthetic,x-snapd.needed-by=/var/lib/kubelet,x-snapd.detach 0 0"
change.go:514: DEBUG: reusing synthetic entry "/var/lib/polkit-1 /var/lib/polkit-1 none rbind,x-snapd.synthetic,x-snapd.needed-by=/var/lib/kubelet,x-snapd.detach 0 0"
change.go:514: DEBUG: reusing synthetic entry "/var/lib/private /var/lib/private none rbind,x-snapd.synthetic,x-snapd.needed-by=/var/lib/kubelet,x-snapd.detach 0 0"
change.go:514: DEBUG: reusing synthetic entry "/var/lib/python /var/lib/python none rbind,x-snapd.synthetic,x-snapd.needed-by=/var/lib/kubelet,x-snapd.detach 0 0"
change.go:514: DEBUG: reusing synthetic entry "/var/lib/snapd /var/lib/snapd none rbind,x-snapd.synthetic,x-snapd.needed-by=/var/lib/kubelet,x-snapd.detach 0 0"
change.go:514: DEBUG: reusing synthetic entry "/var/lib/sudo /var/lib/sudo none rbind,x-snapd.synthetic,x-snapd.needed-by=/var/lib/kubelet,x-snapd.detach 0 0"
change.go:514: DEBUG: reusing synthetic entry "/var/lib/systemd /var/lib/systemd none rbind,x-snapd.synthetic,x-snapd.needed-by=/var/lib/kubelet,x-snapd.detach 0 0"
change.go:514: DEBUG: reusing synthetic entry "/var/lib/ucf /var/lib/ucf none rbind,x-snapd.synthetic,x-snapd.needed-by=/var/lib/kubelet,x-snapd.detach 0 0"
change.go:514: DEBUG: reusing synthetic entry "/var/lib/vim /var/lib/vim none rbind,x-snapd.synthetic,x-snapd.needed-by=/var/lib/kubelet,x-snapd.detach 0 0"
change.go:529: DEBUG: desiredIDs: map[/var/log:true /var/lib/kubelet:true]
change.go:530: DEBUG: reuse: map[/var/lib:true /var/lib/dhcp:true /var/lib/extrausers:true /var/lib/misc:true /var/lib/console-conf:true /var/lib/snapd:true /var/lib/sudo:true /var/lib/dbus:true /var/lib/python:true /var/lib/ucf:true /var/lib/cloud:true /var/lib/pam:true /var/lib/polkit-1:true /var/lib/private:true /var/lib/systemd:true /var/lib/vim:true]
change.go:353: DEBUG: mount --make-rprivate "/var/log" (error: <nil>)
change.go:363: DEBUG: umount "/var/log" UMOUNT_NOFOLLOW|MNT_DETACH (error: <nil>)
change.go:408: DEBUG: remove "/var/log" (error: remove /var/log: read-only file system)
common.go:89: DEBUG: unlocking mount namespace of snap "kubernetes-thing"
common.go:91: DEBUG: thawing processes of snap "kubernetes-thing"
cannot update snap namespace: read-only file system
-----)
```