virtio_blk: verify if queue is broken after virtqueue_get_buf()

In case virtqueue_get_buf() returned with a NULL pointer verify if the
virtqueue is broken in order to leave while loop.
Signed-off-by: Heinz Graalfs <graalfs@linux.vnet.ibm.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

drivers/block/virtio_blk.c

@@ -292,6 +292,8 @@ static void virtblk_done(struct virtqueue *vq)
 				req_done = true;
 			}
 		}
+		if (unlikely(virtqueue_is_broken(vq)))
+			break;
 	} while (!virtqueue_enable_cb(vq));
 	/* In case queue is stopped waiting for more buffers. */
 	if (req_done)