fs: add fastpath for dissolve_on_fput()

Instead of acquiring the namespace semaphore and the mount lock
everytime we close a file with FMODE_NEED_UNMOUNT set add a fastpath
that checks whether we need to at all. Most of the time the caller will
have attached the mount to the filesystem hierarchy and there's nothing
to do.

Link: https://lore.kernel.org/r/20250221-brauner-open_tree-v1-4-dbcfcb98c676@kernel.org


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