ipv6: release dst properly in ipip6_tunnel_xmit

if a dst is not attached to anywhere, it should be released before
exit ipip6_tunnel_xmit, otherwise cause dst memory leakage.

Fixes: 61c1db7f ("ipv6: sit: add GSO/TSO support")
Signed-off-by: NLi RongQing <roy.qing.li@gmail.com>
Acked-by: NEric Dumazet <edumazet@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/sit.c

@@ -966,8 +966,10 @@ static netdev_tx_t ipip6_tunnel_xmit(struct sk_buff *skb,
 	tos = INET_ECN_encapsulate(tos, ipv6_get_dsfield(iph6));
 
 	skb = iptunnel_handle_offloads(skb, false, SKB_GSO_SIT);
-	if (IS_ERR(skb))
+	if (IS_ERR(skb)) {
+		ip_rt_put(rt);
 		goto out;
+	}
 
 	err = iptunnel_xmit(rt, skb, fl4.saddr, fl4.daddr, IPPROTO_IPV6, tos,
 			    ttl, df, !net_eq(tunnel->net, dev_net(dev)));