virtio_balloon: update_balloon_size(): update correct field

According to the virtio spec, the device configuration field
that should be updated after an inflation or deflation
operation is the 'actual' field, not the 'num_pages' one.

Commit 855e0c52 swapped them
in update_balloon_size(). Fix it.
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Fixes: 855e0c52

drivers/virtio/virtio_balloon.c

@@ -285,7 +285,7 @@ static void update_balloon_size(struct virtio_balloon *vb)
 {
 	__le32 actual = cpu_to_le32(vb->num_pages);
 
-	virtio_cwrite(vb->vdev, struct virtio_balloon_config, num_pages,
+	virtio_cwrite(vb->vdev, struct virtio_balloon_config, actual,
 		      &actual);
 }