m_lengthm
pktlen = m_lengthm(m, NULL);
slen = bpf_filter(bpf->bf_insn, (u_char *)m, m_lengthm(m, NULL), 0);
optlen = m_lengthm(exthdrs.ip6e_hbh, NULL) +
m_lengthm(exthdrs.ip6e_dest1, NULL) +
m_lengthm(exthdrs.ip6e_rthdr, NULL);
optlen += m_lengthm(exthdrs.ip6e_dest2, NULL);
u_int m_lengthm(struct mbuf *m, struct mbuf **lastm);