virtio-blk: fix reference a pointer which might be freed

In function virtio_blk_handle_request, it may freed memory pointed by req,
So do not access member of req after calling this function.

Cc: qemu-stable@nongnu.org
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: zhanghailiang <zhang.zhanghailiang@huawei.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

hw/block/virtio-blk.c

@@ -469,8 +469,9 @@ static void virtio_blk_dma_restart_bh(void *opaque)
     s->rq = NULL;
 
     while (req) {
+        VirtIOBlockReq *next = req->next;
         virtio_blk_handle_request(req, &mrb);
-        req = req->next;
+        req = next;
     }
 
     virtio_submit_multiwrite(s->bs, &mrb);