de_cn2bn
*bnp = pmp->pm_rootdirblk + de_cn2bn(pmp, findcn);
bnpercn = de_cn2bn(pmp, 1);
(de_cn2bn(pmp, de_cluster((pmp), (off))))
(de_cn2bn((pmp), (cn)-CLUST_FIRST) + (pmp)->pm_firstcluster)
*bnp = pmp->pm_rootdirblk + de_cn2bn(pmp, findcn);