m_dat
KASSERT(m->m_data == m->m_dat, ("mbuf %p: bad m_data in get", m));
m->m_data = m->m_dat;
m->m_data + m->m_len + n->m_len >= &m->m_dat[MLEN]) {
n->m_data + len < &n->m_dat[MLEN] &&
space = &m->m_dat[MLEN] - (m->m_data + m->m_len);
m->m_data = m->m_dat;
if (m->m_data + m->m_len + sizeof(struct in_addr) >= &m->m_dat[MLEN])
(m)->m_data - (m)->m_dat)
&(m)->m_dat[MLEN] - ((m)->m_data + (m)->m_len))