net: add inet_sk_transparent() helper

Avoids cluttering tcp_v4_send_reset when followup patch extends
it to deal with timewait sockets.

Suggested-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Acked-by: Eric Dumazet <edumazet@google.com>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/request_sock.h

@@ -68,7 +68,7 @@ struct request_sock {
 	u32				peer_secid;
 };
 
-static inline struct request_sock *inet_reqsk(struct sock *sk)
+static inline struct request_sock *inet_reqsk(const struct sock *sk)
 {
 	return (struct request_sock *)sk;
 }

include/net/tcp.h

@@ -1620,6 +1620,18 @@ static inline void tcp_highest_sack_combine(struct sock *sk,
 		tcp_sk(sk)->highest_sack = new;
 }
 
+/* This helper checks if socket has IP_TRANSPARENT set */
+static inline bool inet_sk_transparent(const struct sock *sk)
+{
+	switch (sk->sk_state) {
+	case TCP_TIME_WAIT:
+		return inet_twsk(sk)->tw_transparent;
+	case TCP_NEW_SYN_RECV:
+		return inet_rsk(inet_reqsk(sk))->no_srccheck;
+	}
+	return inet_sk(sk)->transparent;
+}
+
 /* Determines whether this is a thin stream (which may suffer from
  * increased latency). Used to trigger latency-reducing mechanisms.
  */