xen-netfront: convert to GRO API

Anirban was seeing netfront received MTU size packets, which downgraded
throughput. The following patch makes netfront use GRO API which
improves throughput for that case.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Cc: Anirban Chakraborty <abchak@juniper.net>
Cc: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Konrad Wilk <konrad.wilk@oracle.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/xen-netfront.c

@@ -952,7 +952,7 @@ static int handle_incoming_queue(struct net_device *dev,
 		u64_stats_update_end(&stats->syncp);
 
 		/* Pass it up. */
-		netif_receive_skb(skb);
+		napi_gro_receive(&np->napi, skb);
 	}
 
 	return packets_dropped;
@@ -1051,6 +1051,8 @@ err:
 	if (work_done < budget) {
 		int more_to_do = 0;
 
+		napi_gro_flush(napi, false);
+
 		local_irq_save(flags);
 
 		RING_FINAL_CHECK_FOR_RESPONSES(&np->rx, more_to_do);