ip_tunnel: Make none-tunnel-dst tunnel port work with lwtunnel

ip l add dev tun type gretap key 1000
ip a a dev tun 10.0.0.1/24

Packets with tun-id 1000 can be recived by tun dev. But packet can't
be sent through dev tun for non-tunnel-dst

With this patch: tunnel-dst can be get through lwtunnel like beflow:
ip r a 10.0.0.7 encap ip dst 172.168.0.11 dev tun
Signed-off-by: Nwenxu <wenxu@ucloud.cn>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/ip_tunnel.c

@@ -644,13 +644,19 @@ void ip_tunnel_xmit(struct sk_buff *skb, struct net_device *dev,
 	dst = tnl_params->daddr;
 	if (dst == 0) {
 		/* NBMA tunnel */
+		struct ip_tunnel_info *tun_info;
 
 		if (!skb_dst(skb)) {
 			dev->stats.tx_fifo_errors++;
 			goto tx_error;
 		}
 
-		if (skb->protocol == htons(ETH_P_IP)) {
+		tun_info = skb_tunnel_info(skb);
+		if (tun_info && (tun_info->mode & IP_TUNNEL_INFO_TX) &&
+		    ip_tunnel_info_af(tun_info) == AF_INET &&
+		    tun_info->key.u.ipv4.dst)
+			dst = tun_info->key.u.ipv4.dst;
+		else if (skb->protocol == htons(ETH_P_IP)) {
 			rt = skb_rtable(skb);
 			dst = rt_nexthop(rt, inner_iph->daddr);
 		}