v4l2: v4l2_camera_proxy: Acquire only one buffer semaphore on VIDIOC_DQBUF

We use a semaphore to atomically keep track of how many buffers are
available for dequeueing. The check for how to acquire the semaphore was
incorrect, leading to a double acquire upon a successful nonblocking
acquire. Fix this.

Signed-off-by: Paul Elder <paul.elder@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>

src/v4l2/v4l2_camera_proxy.cpp

@@ -426,10 +426,10 @@ int V4L2CameraProxy::vidioc_dqbuf(struct v4l2_buffer *arg)
 	    !validateMemoryType(arg->memory))
 		return -EINVAL;
 
-	if (nonBlocking_ && !vcam_->bufferSema_.tryAcquire())
-		return -EAGAIN;
-	else
+	if (!nonBlocking_)
 		vcam_->bufferSema_.acquire();
+	else if (!vcam_->bufferSema_.tryAcquire())
+		return -EAGAIN;
 
 	updateBuffers();