tcp_signature
if (tcp_signature(m, th, toff, sav, sig) < 0) {
tcp_signature(m, th, (char *)th - mtod(m, char *), sav, sigp);
(void)tcp_signature(m, th, hlen, sav, sigp);
int tcp_signature(struct mbuf *, struct tcphdr *, int, struct secasvar *,