fs: add mount namespace to rbtree late

fs: add mount namespace to rbtree late

There's no point doing that under the namespace semaphore it just gives
the false impression that it protects the mount namespace rbtree and it
simply doesn't.

Link: https://lore.kernel.org/r/20241213-work-mount-rbtree-lockless-v3-2-6e3cdaf9b280@kernel.org
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>