RT_XROUNDUP
cp += RT_XROUNDUP(sa->sa_len);
cp - RT_XROUNDUP(sa->sa_len) + sa->sa_len == cplim)
dlen = RT_XROUNDUP(sa->sa_len);
dlen = RT_XROUNDUP(sa->sa_len);
cp += RT_XROUNDUP(sa->sa_len);