sit: use dst_cache in ipip6_tunnel_xmit

Same as other ip tunnel, use dst_cache in xmit action to avoid
unnecessary fib lookups.
Signed-off-by: NHaishuang Yan <yanhaishuang@cmss.chinamobile.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/sit.c

@@ -900,12 +900,17 @@ static netdev_tx_t ipip6_tunnel_xmit(struct sk_buff *skb,
 			   RT_TOS(tos), RT_SCOPE_UNIVERSE, IPPROTO_IPV6,
 			   0, dst, tiph->saddr, 0, 0,
 			   sock_net_uid(tunnel->net, NULL));
-	rt = ip_route_output_flow(tunnel->net, &fl4, NULL);
 
-	if (IS_ERR(rt)) {
-		dev->stats.tx_carrier_errors++;
-		goto tx_error_icmp;
+	rt = dst_cache_get_ip4(&tunnel->dst_cache, &fl4.saddr);
+	if (!rt) {
+		rt = ip_route_output_flow(tunnel->net, &fl4, NULL);
+		if (IS_ERR(rt)) {
+			dev->stats.tx_carrier_errors++;
+			goto tx_error_icmp;
+		}
+		dst_cache_set_ip4(&tunnel->dst_cache, &rt->dst, fl4.saddr);
 	}
+
 	if (rt->rt_type != RTN_UNICAST) {
 		ip_rt_put(rt);
 		dev->stats.tx_carrier_errors++;