media: Clear devnode->media_dev holding media_devnode_lock

Fix media_devnode_unregister() to clear devnode->media_dev while holding
media_devnode_lock. media_devnode_register()'s cdev_add_error handling
does this correctly.
Signed-off-by: Shuah Khan <shuah@kernel.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

drivers/media/media-devnode.c

@@ -291,8 +291,9 @@ void media_devnode_unregister(struct media_devnode *devnode)
 	mutex_lock(&media_devnode_lock);
 	/* Delete the cdev on this minor as well */
 	cdev_device_del(&devnode->cdev, &devnode->dev);
-	mutex_unlock(&media_devnode_lock);
 	devnode->media_dev = NULL;
+	mutex_unlock(&media_devnode_lock);
+
 	put_device(&devnode->dev);
 }