wbuflen
wlen = wbuflen(&ms->mbstate, buf, len);
arc_msgbuf(struct arc_softc *sc, void *wptr, size_t wbuflen, void *rptr,
device_xname(sc->sc_dev), wbuflen, rbuflen);
wlen = sizeof(struct arc_fw_bufhdr) + wbuflen + 1; /* 1 for cksum */
bufhdr->len = htole16(wbuflen);
memcpy(wbuf + sizeof(struct arc_fw_bufhdr), wptr, wbuflen);
wbuf[wlen - 1] = arc_msg_cksum(wptr, wbuflen);