virtio_scsi: verify if queue is broken after virtqueue_get_buf()

If virtqueue_get_buf() returned with a NULL pointer avoid a possibly
endless loop by checking for a broken virtqueue.
Signed-off-by: Heinz Graalfs <graalfs@linux.vnet.ibm.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

drivers/scsi/virtio_scsi.c

@@ -224,6 +224,9 @@ static void virtscsi_vq_done(struct virtio_scsi *vscsi,
 		virtqueue_disable_cb(vq);
 		while ((buf = virtqueue_get_buf(vq, &len)) != NULL)
 			fn(vscsi, buf);
+
+		if (unlikely(virtqueue_is_broken(vq)))
+			break;
 	} while (!virtqueue_enable_cb(vq));
 	spin_unlock_irqrestore(&virtscsi_vq->vq_lock, flags);
 }