fsnotify: Provide framework for dropping SRCU lock in ->handle_event

fanotify wants to drop fsnotify_mark_srcu lock when waiting for response
from userspace so that the whole notification subsystem is not blocked
during that time. This patch provides a framework for safely getting
mark reference for a mark found in the object list which pins the mark
in that list. We can then drop fsnotify_mark_srcu, wait for userspace
response and then safely continue iteration of the object list once we
reaquire fsnotify_mark_srcu.

Reviewed-by: Miklos Szeredi <mszeredi@redhat.com>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>

fs/notify/group.c

@@ -126,6 +126,7 @@ struct fsnotify_group *fsnotify_alloc_group(const struct fsnotify_ops *ops)
 	/* set to 0 when there a no external references to this group */
 	atomic_set(&group->refcnt, 1);
 	atomic_set(&group->num_marks, 0);
+	atomic_set(&group->user_waits, 0);
 
 	spin_lock_init(&group->notification_lock);
 	INIT_LIST_HEAD(&group->notification_list);