mount: ensure we don't pointlessly walk the mount tree

This logic got broken recently. Add it back.

Fixes: 474f7825 ("fs: add copy_mount_setattr() helper")
Link: https://lore.kernel.org/20250409-sektflaschen-gecko-27c021fbd222@brauner


Tested-by: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>