tcp: use inet6_csk_route_req() in tcp_v6_send_synack()

	struct inet6_request_sock *treq = inet6_rsk(req);

	struct ipv6_pinfo *np = inet6_sk(sk);

	struct sk_buff * skb;

	struct ipv6_txoptions *opt = NULL;

	struct in6_addr * final_p, final;

	struct ipv6_txoptions *opt = np->opt;

	struct flowi6 fl6;

	struct dst_entry *dst;

	int err;

	memset(&fl6, 0, sizeof(fl6));

	fl6.flowi6_proto = IPPROTO_TCP;

	fl6.daddr = treq->rmt_addr;

	fl6.saddr = treq->loc_addr;

	fl6.flowlabel = 0;

	fl6.flowi6_oif = treq->iif;

	fl6.flowi6_mark = sk->sk_mark;

	fl6.fl6_dport = inet_rsk(req)->rmt_port;

	fl6.fl6_sport = inet_rsk(req)->loc_port;

	security_req_classify_flow(req, flowi6_to_flowi(&fl6));

	opt = np->opt;

	final_p = fl6_update_dst(&fl6, opt, &final);

	int err = -ENOMEM;

	dst = ip6_dst_lookup_flow(sk, &fl6, final_p, false);

	if (IS_ERR(dst)) {

		err = PTR_ERR(dst);

		dst = NULL;

	dst = inet6_csk_route_req(sk, &fl6, req);

	if (!dst)

		goto done;

	}

	skb = tcp_make_synack(sk, dst, req, rvp);

	err = -ENOMEM;

	if (skb) {

		__tcp_v6_send_check(skb, &treq->loc_addr, &treq->rmt_addr);