tcp: md5: only call tp->af_specific->md5_lookup() for md5 sockets

RETPOLINE made calls to tp->af_specific->md5_lookup() quite expensive,
given they have no result.
We can omit the calls for sockets that have no md5 keys.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_output.c

@@ -585,14 +585,15 @@ static unsigned int tcp_syn_options(struct sock *sk, struct sk_buff *skb,
 	unsigned int remaining = MAX_TCP_OPTION_SPACE;
 	struct tcp_fastopen_request *fastopen = tp->fastopen_req;
 
+	*md5 = NULL;
 #ifdef CONFIG_TCP_MD5SIG
+	if (unlikely(rcu_access_pointer(tp->md5sig_info))) {
 		*md5 = tp->af_specific->md5_lookup(sk, sk);
 		if (*md5) {
 			opts->options |= OPTION_MD5;
 			remaining -= TCPOLEN_MD5SIG_ALIGNED;
 		}
-#else
-	*md5 = NULL;
+	}
 #endif
 
 	/* We always get an MSS option.  The option bytes which will be seen in
@@ -720,14 +721,15 @@ static unsigned int tcp_established_options(struct sock *sk, struct sk_buff *skb
 
 	opts->options = 0;
 
+	*md5 = NULL;
 #ifdef CONFIG_TCP_MD5SIG
+	if (unlikely(rcu_access_pointer(tp->md5sig_info))) {
 		*md5 = tp->af_specific->md5_lookup(sk, sk);
-	if (unlikely(*md5)) {
+		if (*md5) {
 			opts->options |= OPTION_MD5;
 			size += TCPOLEN_MD5SIG_ALIGNED;
 		}
-#else
-	*md5 = NULL;
+	}
 #endif
 
 	if (likely(tp->rx_opt.tstamp_ok)) {