[media] media-devnode: add missing mutex lock in error handler

We should protect the device unregister patch too, at the error
condition.
Signed-off-by: NMax Kellermann <max@duempel.org>
Signed-off-by: NMauro Carvalho Chehab <mchehab@osg.samsung.com>

drivers/media/media-devnode.c

@@ -267,8 +267,11 @@ int __must_check media_devnode_register(struct media_devnode *mdev,
 	return 0;
 
 error:
+	mutex_lock(&media_devnode_lock);
 	cdev_del(&mdev->cdev);
 	clear_bit(mdev->minor, media_devnode_nums);
+	mutex_unlock(&media_devnode_lock);
+
 	return ret;
 }