m_makespace
struct mbuf *m_makespace(struct mbuf *, int, int, int *);
mo = m_makespace(m, sizeof(struct ip), hlen, &roff);
mo = m_makespace(m, sizeof(struct ip6_hdr), hlen, &roff);
mi = m_makespace(m, skip, ahsize, &roff);
mo = m_makespace(m, skip, hlen, &roff);
mo = m_makespace(m, skip, IPCOMP_HLENGTH, &roff);