selftests/filesystems: check that a copied mount namespace keeps unbindable

Make a tmpfs unbindable, copy the mount namespace and check from the
copy that:

- a bind of the mount fails with EINVAL

selftests/filesystems: check that a copied mount namespace keeps unbindable

Make a tmpfs unbindable, copy the mount namespace and check from the
copy that:

- a bind of the mount fails with EINVAL

- a recursive bind fails as well

- open_tree(OPEN_TREE_CLONE) of it fails

- mountinfo still shows it as unbindable

- a copy of the copy refuses the bind too

The first case runs in the original namespace so the fixture stays
honest about what it set up.

Link: https://patch.msgid.link/20260923-work-mount-fixes-v1-2-f424cf8d3242@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

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