virtio-blk: Fix double completion for werror=stop

When a request R is absorbed by request M, it is appended to the
"mr_next" queue led by M, and is completed together with the completion
of M, in virtio_blk_rw_complete.

During DMA restart in virtio_blk_dma_restart_bh, requests in s->rq are
parsed and submitted again, possibly with a stale req->mr_next. It could
be a problem if the request merging in virtio_blk_handle_request hasn't
refreshed every mr_next pointer, in which case, virtio_blk_rw_complete
could walk through unexpected requests following the stale pointers.

Fix this by unsetting the pointer in virtio_blk_rw_complete. It is safe
because this req is either completed and freed right away, or it will be
restarted and parsed from scratch out of the vq later.

Signed-off-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

hw/block/virtio-blk.c

@@ -112,6 +112,10 @@ static void virtio_blk_rw_complete(void *opaque, int ret)
              * happen on the other side of the migration).
              */
             if (virtio_blk_handle_rw_error(req, -ret, is_read)) {
+                /* Break the link in case the next request is added to the
+                 * restart queue and is going to be parsed from the ring again.
+                 */
+                req->mr_next = NULL;
                 continue;
             }
         }