virtio_net: fix return value check in receive_mergeable()

The function virtqueue_get_buf_ctx() could return NULL, the return
value 'buf' need to be checked with NULL, not value 'ctx'.
Signed-off-by: NYunjian Wang <wangyunjian@huawei.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

drivers/net/virtio_net.c

@@ -714,7 +714,7 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
 		int num_skb_frags;
 
 		buf = virtqueue_get_buf_ctx(rq->vq, &len, &ctx);
-		if (unlikely(!ctx)) {
+		if (unlikely(!buf)) {
 			pr_debug("%s: rx error: %d buffers out of %d missing\n",
 				 dev->name, num_buf,
 				 virtio16_to_cpu(vi->vdev,