tcp_ipv6: fix use of uninitialized memory

inet6_rsk() is called on a struct request_sock * before we
have checked whether the socket is an ipv6 socket or a ipv6-
mapped ipv4 socket. The access that triggers this is the
inet_rsk(rsk)->inet6_rsk_offset dereference in inet6_rsk().

This is arguably not a critical error as the inet6_rsk_offset
is only used to compute a pointer which is never really used
(in the code path in question) anyway. But it might be a
latent error, so let's fix it.

Spotted by kmemcheck.
Signed-off-by: Vegard Nossum <vegard.nossum@gmail.com>
Acked-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/tcp_ipv6.c

@@ -1286,7 +1286,7 @@ static struct sock * tcp_v6_syn_recv_sock(struct sock *sk, struct sk_buff *skb,
 					  struct request_sock *req,
 					  struct dst_entry *dst)
 {
-	struct inet6_request_sock *treq = inet6_rsk(req);
+	struct inet6_request_sock *treq;
 	struct ipv6_pinfo *newnp, *np = inet6_sk(sk);
 	struct tcp6_sock *newtcp6sk;
 	struct inet_sock *newinet;
@@ -1350,6 +1350,7 @@ static struct sock * tcp_v6_syn_recv_sock(struct sock *sk, struct sk_buff *skb,
 		return newsk;
 	}
 
+	treq = inet6_rsk(req);
 	opt = np->opt;
 
 	if (sk_acceptq_is_full(sk))