tcp: use correct net ns in cookie_v4_check()

Its better to make a route lookup in appropriate namespace.
Signed-off-by: NEric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/syncookies.c

@@ -347,7 +347,7 @@ struct sock *cookie_v4_check(struct sock *sk, struct sk_buff *skb,
 					       { .sport = th->dest,
 						 .dport = th->source } } };
 		security_req_classify_flow(req, &fl);
-		if (ip_route_output_key(&init_net, &rt, &fl)) {
+		if (ip_route_output_key(sock_net(sk), &rt, &fl)) {
 			reqsk_free(req);
 			goto out;
 		}