mb_reserve
rqp->sr_rqsig = (u_int8_t *)mb_reserve(mbp, 8);
rqp->sr_rqtid = mb_reserve(mbp, sizeof(u_int16_t));
rqp->sr_rquid = mb_reserve(mbp, sizeof(u_int16_t));
rqp->sr_wcount = mb_reserve(&rqp->sr_rq, sizeof(u_int8_t));
rqp->sr_bcount = mb_reserve(&rqp->sr_rq, sizeof(u_short));
caddr_t mb_reserve(struct mbchain *mbp, int size);