fsnotify: let a filesystem refuse marks on its objects

Don't let nullfs be watched. fanotify refuses mount and filesystem marks
on SB_NOUSER superblocks but inode marks of inotify, fanotify and
dnotify go through. The one inode of knullfs is the root of every
kernel thread and the following patches make it reachable from
userspace as the directory that stands in for an unmounted mount. A
watch placed through one such directory would report the opens through
all the others, across users.

Add FS_DISALLOW_NOTIFY next to FS_DISALLOW_NOTIFY_PERM, refuse a mark on
any object of such a filesystem in fsnotify_add_mark_list() where every
backend ends up and set it for nullfs. There's nothing to watch on a
permanently empty and immutable filesystem.

Link: https://patch.msgid.link/20261002-work-mount-fixes-4-v1-16-dd44b89d44ce@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

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