virtio-balloon: improve update_balloon_size_func

There is no need to update the balloon actual register when there is no
ballooning request. This patch avoids update_balloon_size when diff is 0.
Signed-off-by: Wei Wang <wei.w.wang@intel.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Halil Pasic <pasic@linux.ibm.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/virtio/virtio_balloon.c

@@ -457,9 +457,12 @@ static void update_balloon_size_func(struct work_struct *work)
 			  update_balloon_size_work);
 	diff = towards_target(vb);
 
+	if (!diff)
+		return;
+
 	if (diff > 0)
 		diff -= fill_balloon(vb, diff);
-	else if (diff < 0)
+	else
 		diff += leak_balloon(vb, -diff);
 	update_balloon_size(vb);