selinux: add FILE__WATCH_MOUNTNS

Watching mount namespaces for changes (mount, umount, move mount) was added
by previous patches.

This patch adds the file/watch_mountns permission that can be applied to
nsfs files (/proc/$$/ns/mnt), making it possible to allow or deny watching
a particular namespace for changes.

Suggested-by: Paul Moore <paul@paul-moore.com>
Link: https://lore.kernel.org/all/CAHC9VhTOmCjCSE2H0zwPOmpFopheexVb6jyovz92ZtpKtoVv6A@mail.gmail.com/


Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Link: https://lore.kernel.org/r/20250224154836.958915-1-mszeredi@redhat.com


Acked-by: Paul Moore <paul@paul-moore.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>