M_SIZE
#define M_TRAILINGROOM(m) ((M_START(m) + M_SIZE(m)) - ((m)->m_data + (m)->m_len))
adjust = M_SIZE(m) - len;
len -= M_SIZE(mb);
b = a + M_SIZE(m);
if (len < M_SIZE(m))
KASSERT(M_DATABUF(m) + M_SIZE(m) >= (m->m_data + m->m_len));
return M_DATABUF(m) + M_SIZE(m) - (m->m_data + m->m_len);