net: Use ip_route_input_noref() in input path

Use ip_route_input_noref() in ip fast path, to avoid two atomic ops per
incoming packet.

Note: loopback is excluded from this optimization in ip_rcv_finish()

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv4/ip_input.c b/net/ipv4/ip_input.c
index af76de5..d930dc5 100644
--- a/net/ipv4/ip_input.c
+++ b/net/ipv4/ip_input.c
@@ -331,8 +331,8 @@
 	 *	how the packet travels inside Linux networking.
 	 */
 	if (skb_dst(skb) == NULL) {
-		int err = ip_route_input(skb, iph->daddr, iph->saddr, iph->tos,
-					 skb->dev);
+		int err = ip_route_input_noref(skb, iph->daddr, iph->saddr,
+					       iph->tos, skb->dev);
 		if (unlikely(err)) {
 			if (err == -EHOSTUNREACH)
 				IP_INC_STATS_BH(dev_net(skb->dev),