m_makespace
struct mbuf *m_makespace(struct mbuf *m0, int skip, int hlen, int *off);
n = m_makespace(m, hlen, sizeof(*udp), &off);
mi = m_makespace(m, skip, ahsize, &roff);
mo = m_makespace(m, skip, hlen, &roff);
mo = m_makespace(m, skip, IPCOMP_HLENGTH, &roff);