ipv6: syncookies: do not skip ->iif initialization

When syncookies are in effect, req->iif is left uninitialized.
In case of e.g. link-local addresses the route lookup then fails
and no syn-ack is sent.

Rearrange things so ->iif is also initialized in the syncookie case.

want_cookie can only be true when the isn was zero, thus move the want_cookie
check into the "!isn" branch.

Cc: Glenn Griffin <ggriffin.kernel@gmail.com>
Signed-off-by: NFlorian Westphal <fw@strlen.de>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/tcp_ipv6.c

@@ -1272,10 +1272,7 @@ static int tcp_v6_conn_request(struct sock *sk, struct sk_buff *skb)
 	if (!want_cookie)
 		TCP_ECN_create_request(req, tcp_hdr(skb));
 
-	if (want_cookie) {
-		isn = cookie_v6_init_sequence(sk, skb, &req->mss);
-		req->cookie_ts = tmp_opt.tstamp_ok;
-	} else if (!isn) {
+	if (!isn) {
 		if (ipv6_opt_accepted(sk, skb) ||
 		    np->rxopt.bits.rxinfo || np->rxopt.bits.rxoinfo ||
 		    np->rxopt.bits.rxhlim || np->rxopt.bits.rxohlim) {
@@ -1288,8 +1285,12 @@ static int tcp_v6_conn_request(struct sock *sk, struct sk_buff *skb)
 		if (!sk->sk_bound_dev_if &&
 		    ipv6_addr_type(&treq->rmt_addr) & IPV6_ADDR_LINKLOCAL)
 			treq->iif = inet6_iif(skb);
-
+		if (!want_cookie) {
 			isn = tcp_v6_init_sequence(skb);
+		} else {
+			isn = cookie_v6_init_sequence(sk, skb, &req->mss);
+			req->cookie_ts = tmp_opt.tstamp_ok;
+		}
 	}
 	tcp_rsk(req)->snt_isn = isn;