fsnotify: add mount notification infrastructure

This is just the plumbing between the event source (fs/namespace.c) and the
event consumer (fanotify).  In itself it does nothing.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Link: https://lore.kernel.org/r/20250129165803.72138-2-mszeredi@redhat.com


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