fs: add assert for move_mount()

After we've attached a detached mount tree the anonymous mount namespace
must be empty. Add an assert and make this assumption explicit.

Link: https://lore.kernel.org/r/20250221-brauner-open_tree-v1-3-dbcfcb98c676@kernel.org


Signed-off-by: Christian Brauner <brauner@kernel.org>