net/tcp: Add a helper tcp_ao_hdr_maclen()

It's going to be used more in TCP-AO tracepoints.
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Dmitry Safonov <0x7f454c46@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/tcp_ao.h

@@ -19,6 +19,11 @@ struct tcp_ao_hdr {
 	u8	rnext_keyid;
 };
 
+static inline u8 tcp_ao_hdr_maclen(const struct tcp_ao_hdr *aoh)
+{
+	return aoh->length - sizeof(struct tcp_ao_hdr);
+}
+
 struct tcp_ao_counters {
 	atomic64_t	pkt_good;
 	atomic64_t	pkt_bad;

net/ipv4/tcp_ao.c

@@ -884,8 +884,8 @@ tcp_ao_verify_hash(const struct sock *sk, const struct sk_buff *skb,
 		   const struct tcp_ao_hdr *aoh, struct tcp_ao_key *key,
 		   u8 *traffic_key, u8 *phash, u32 sne, int l3index)
 {
-	u8 maclen = aoh->length - sizeof(struct tcp_ao_hdr);
 	const struct tcphdr *th = tcp_hdr(skb);
+	u8 maclen = tcp_ao_hdr_maclen(aoh);
 	void *hash_buf = NULL;
 
 	if (maclen != tcp_ao_maclen(key)) {