selftests/filesystems: check that a lock lands on the right mount and stays

Check MNT_LOCKED:

- A child in a user namespace of its own triggers the tracefs automount
  through a directory descriptor on the host's debugfs mount. The copy
  in the child's namespace has to be locked and the host's mount unlocked.

- A child moves a bind of that automount beneath a locked covering mount,
  unmounts the covering mount and asks for the umount of an unlocked
  ancestor. This must not expire the mount which holds the lock now.

- The host mounts and unmounts on a directory that a locked mount covers
  in a user namespace further down the propagation chain. That cover has
  to be locked afterwards as it was before.

Link: https://patch.msgid.link/20261002-work-mount-fixes-4-v1-10-dd44b89d44ce@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

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