selftests/filesystems: test that a mntns file can't take over a lock

Add a case to the locked mount test:

- a bind of the mount namespace file of a grandchild moved beneath a
  locked cover must fail with EINVAL
- a bind of an ordinary file beneath it takes the lock over, the old
  cover can go and the new one can't
- the file is still covered after unshare(CLONE_NEWNS)

Link: https://patch.msgid.link/20261014-work-namei-locked-mnt_ns_file-v1-2-24035bd5d8e9@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>