[media] v4l: vb2: Fix race condition in _vb2_fop_release

The function releases the queue if the file being released is the queue
owner. The check reads the queue->owner field without taking the queue
lock, creating a race condition with functions that set the queue owner,
such as vb2_ioctl_reqbufs() for instance.

Fix this by moving the queue->owner check within the mutex protected
section.
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>

drivers/media/v4l2-core/videobuf2-core.c

@@ -3389,14 +3389,14 @@ int _vb2_fop_release(struct file *file, struct mutex *lock)
 {
 	struct video_device *vdev = video_devdata(file);
 
-	if (file->private_data == vdev->queue->owner) {
 	if (lock)
 		mutex_lock(lock);
+	if (file->private_data == vdev->queue->owner) {
 		vb2_queue_release(vdev->queue);
 		vdev->queue->owner = NULL;
+	}
 	if (lock)
 		mutex_unlock(lock);
-	}
 	return v4l2_fh_release(file);
 }
 EXPORT_SYMBOL_GPL(_vb2_fop_release);