media: v4l: async: Correctly serialise async sub-device unregistration

The check whether an async sub-device is bound to a notifier was performed
without list_lock held, making it possible for another process to
unbind the async sub-device before the sub-device unregistration function
proceeds to take the lock.

Fix this by first acquiring the lock and then proceeding with the check.
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

drivers/media/v4l2-core/v4l2-async.c

@@ -298,20 +298,16 @@ EXPORT_SYMBOL(v4l2_async_register_subdev);
 
 void v4l2_async_unregister_subdev(struct v4l2_subdev *sd)
 {
-	struct v4l2_async_notifier *notifier = sd->notifier;
-
-	if (!sd->asd) {
-		if (!list_empty(&sd->async_list))
-			v4l2_async_cleanup(sd);
-		return;
-	}
-
 	mutex_lock(&list_lock);
 
+	if (sd->asd) {
+		struct v4l2_async_notifier *notifier = sd->notifier;
+
 		list_add(&sd->asd->list, &notifier->waiting);
 
 		if (notifier->unbind)
 			notifier->unbind(notifier, sd, sd->asd);
+	}
 
 	v4l2_async_cleanup(sd);