vfs: add notifications for mount attach and detach

Add notifications for attaching and detaching mounts to fs/namespace.c

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Link: https://lore.kernel.org/r/20250129165803.72138-4-mszeredi@redhat.com


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