ipv6: convert rx data path to not take refcnt on dst

ip6_route_input() is the key function to do the route lookup in the
rx data path. All the callers to this function are already holding rcu
lock. So it is fairly easy to convert it to not take refcnt on the dst:
We pass in flag RT6_LOOKUP_F_DST_NOREF and do skb_dst_set_noref().
This saves a few atomic inc or dec operations and should boost
performance overall.
This also makes the logic more aligned with v4.
Signed-off-by: Wei Wang <weiwan@google.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Acked-by: Mahesh Bandewar <maheshb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/route.c

@@ -2375,11 +2375,12 @@ u32 rt6_multipath_hash(const struct net *net, const struct flowi6 *fl6,
 	return mhash >> 1;
 }
 
+/* Called with rcu held */
 void ip6_route_input(struct sk_buff *skb)
 {
 	const struct ipv6hdr *iph = ipv6_hdr(skb);
 	struct net *net = dev_net(skb->dev);
-	int flags = RT6_LOOKUP_F_HAS_SADDR;
+	int flags = RT6_LOOKUP_F_HAS_SADDR | RT6_LOOKUP_F_DST_NOREF;
 	struct ip_tunnel_info *tun_info;
 	struct flowi6 fl6 = {
 		.flowi6_iif = skb->dev->ifindex,
@@ -2401,8 +2402,8 @@ void ip6_route_input(struct sk_buff *skb)
 	if (unlikely(fl6.flowi6_proto == IPPROTO_ICMPV6))
 		fl6.mp_hash = rt6_multipath_hash(net, &fl6, skb, flkeys);
 	skb_dst_drop(skb);
-	skb_dst_set(skb,
-		    ip6_route_input_lookup(net, skb->dev, &fl6, skb, flags));
+	skb_dst_set_noref(skb, ip6_route_input_lookup(net, skb->dev,
+						      &fl6, skb, flags));
 }
 
 static struct rt6_info *ip6_pol_route_output(struct net *net,