net/tcp: Only produce AO/MD5 logs if there are any keys

User won't care about inproper hash options in the TCP header if they
don't use neither TCP-AO nor TCP-MD5. Yet, those logs can add up in
syslog, while not being a real concern to the host admin:
> kernel: TCP: TCP segment has incorrect auth options set for XX.20.239.12.54681->XX.XX.90.103.80 [S]

Keep silent and avoid logging when there aren't any keys in the system.

Side-note: I also defined static_branch_tcp_*() helpers to avoid more
ifdeffery, going to remove more ifdeffery further with their help.

Reported-by: Christian Kujau <lists@nerdbynature.de>
Closes: https://lore.kernel.org/all/f6b59324-1417-566f-a976-ff2402718a8d@nerdbynature.de/
Signed-off-by: Dmitry Safonov <dima@arista.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Fixes: 2717b5a ("net/tcp: Add tcp_hash_fail() ratelimited logs")
Link: https://lore.kernel.org/r/20240104-tcp_hash_fail-logs-v1-1-ff3e1f6f9e72@arista.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/net/tcp.h

@@ -1788,8 +1788,6 @@ struct tcp_md5sig_key *tcp_v4_md5_lookup(const struct sock *sk,
 					 const struct sock *addr_sk);
 
 #ifdef CONFIG_TCP_MD5SIG
-#include <linux/jump_label.h>
-extern struct static_key_false_deferred tcp_md5_needed;
 struct tcp_md5sig_key *__tcp_md5_do_lookup(const struct sock *sk, int l3index,
 					   const union tcp_md5_addr *addr,
 					   int family, bool any_l3index);

include/net/tcp_ao.h

@@ -127,12 +127,35 @@ struct tcp_ao_info {
 	struct rcu_head		rcu;
 };
 
+#ifdef CONFIG_TCP_MD5SIG
+#include <linux/jump_label.h>
+extern struct static_key_false_deferred tcp_md5_needed;
+#define static_branch_tcp_md5()	static_branch_unlikely(&tcp_md5_needed.key)
+#else
+#define static_branch_tcp_md5()	false
+#endif
+#ifdef CONFIG_TCP_AO
+/* TCP-AO structures and functions */
+#include <linux/jump_label.h>
+extern struct static_key_false_deferred tcp_ao_needed;
+#define static_branch_tcp_ao()	static_branch_unlikely(&tcp_ao_needed.key)
+#else
+#define static_branch_tcp_ao()	false
+#endif
+
+static inline bool tcp_hash_should_produce_warnings(void)
+{
+	return static_branch_tcp_md5() || static_branch_tcp_ao();
+}
+
 #define tcp_hash_fail(msg, family, skb, fmt, ...)			\
 do {									\
 	const struct tcphdr *th = tcp_hdr(skb);				\
 	char hdr_flags[6];						\
 	char *f = hdr_flags;						\
 									\
+	if (!tcp_hash_should_produce_warnings())			\
+		break;							\
 	if (th->fin)							\
 		*f++ = 'F';						\
 	if (th->syn)							\
@@ -159,9 +182,6 @@ do {									\
 
 #ifdef CONFIG_TCP_AO
 /* TCP-AO structures and functions */
-#include <linux/jump_label.h>
-extern struct static_key_false_deferred tcp_ao_needed;
-
 struct tcp4_ao_context {
 	__be32		saddr;
 	__be32		daddr;