virtio: fix net driver loop case where we fail to restart

skb is only NULL the first time around: it's more correct to test for
being under-budget.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

drivers/net/virtio_net.c

@@ -198,8 +198,8 @@ static int virtnet_poll(struct napi_struct *napi, int budget)
 	if (vi->num < vi->max / 2)
 		try_fill_recv(vi);
 
-	/* All done? */
-	if (!skb) {
+	/* Out of packets? */
+	if (received < budget) {
 		netif_rx_complete(vi->dev, napi);
 		if (unlikely(!vi->rvq->vq_ops->restart(vi->rvq))
 		    && netif_rx_reschedule(vi->dev, napi))