virtio-net: correct error handling of virtqueue_kick()

Current error handling of virtqueue_kick() was wrong in two places:
- The skb were freed immediately when virtqueue_kick() fail during
  xmit. This may lead double free since the skb was not detached from
  the virtqueue.
- try_fill_recv() returns false when virtqueue_kick() fail. This will
  lead unnecessary rescheduling of refill work.

Actually, it's safe to just ignore the kick failure in those two
places. So this patch fixes this by partially revert commit
6797590.

Fixes 6797590
(virtio_net: verify if virtqueue_kick() succeeded).

Cc: Heinz Graalfs <graalfs@linux.vnet.ibm.com>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/virtio_net.c

@@ -671,8 +671,7 @@ static bool try_fill_recv(struct receive_queue *rq, gfp_t gfp)
 		if (err)
 			break;
 	} while (rq->vq->num_free);
-	if (unlikely(!virtqueue_kick(rq->vq)))
-		return false;
+	virtqueue_kick(rq->vq);
 	return !oom;
 }
 
@@ -877,7 +876,7 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
 	err = xmit_skb(sq, skb);
 
 	/* This should not happen! */
-	if (unlikely(err) || unlikely(!virtqueue_kick(sq->vq))) {
+	if (unlikely(err)) {
 		dev->stats.tx_fifo_errors++;
 		if (net_ratelimit())
 			dev_warn(&dev->dev,
@@ -886,6 +885,7 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
 		kfree_skb(skb);
 		return NETDEV_TX_OK;
 	}
+	virtqueue_kick(sq->vq);
 
 	/* Don't wait up for transmitted skbs to be freed. */
 	skb_orphan(skb);