ipv6: inet6_sk() should use sk_fullsock()

SYN_RECV & TIMEWAIT sockets are not full blown, they do not have a pinet6
pointer.

Fixes: ca6fb06 ("tcp: attach SYNACK messages to request sockets instead of listener")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/ipv6.h

@@ -264,9 +264,9 @@ struct tcp6_timewait_sock {
 };
 
 #if IS_ENABLED(CONFIG_IPV6)
-static inline struct ipv6_pinfo * inet6_sk(const struct sock *__sk)
+static inline struct ipv6_pinfo *inet6_sk(const struct sock *__sk)
 {
-	return inet_sk(__sk)->pinet6;
+	return sk_fullsock(__sk) ? inet_sk(__sk)->pinet6 : NULL;
 }
 
 static inline struct raw6_sock *raw6_sk(const struct sock *sk)