fsnotify: Allocate overflow events with proper type

Commit 7053aee26a "fsnotify: do not share events between notification
groups" used overflow event statically allocated in a group with the
size of the generic notification event. This causes problems because
some code looks at type specific parts of event structure and gets
confused by a random data it sees there and causes crashes.

Fix the problem by allocating overflow event with type corresponding to
the group type so code cannot get confused.

Signed-off-by: Jan Kara <jack@suse.cz>

fs/notify/group.c

@@ -55,6 +55,13 @@ void fsnotify_destroy_group(struct fsnotify_group *group)
 	/* clear the notification queue of all events */
 	fsnotify_flush_notify(group);
 
+	/*
+	 * Destroy overflow event (we cannot use fsnotify_destroy_event() as
+	 * that deliberately ignores overflow events.
+	 */
+	if (group->overflow_event)
+		group->ops->free_event(group->overflow_event);
+
 	fsnotify_put_group(group);
 }
 
@@ -99,7 +106,6 @@ struct fsnotify_group *fsnotify_alloc_group(const struct fsnotify_ops *ops)
 	INIT_LIST_HEAD(&group->marks_list);
 
 	group->ops = ops;
-	fsnotify_init_event(&group->overflow_event, NULL, FS_Q_OVERFLOW);
 
 	return group;
 }