db_maxlevel
if ((rc = dbFindCtl(bmp, l2nb, bmp->db_maxlevel, &blkno)))
bmp->db_maxlevel = le32_to_cpu(dbmp_le->dn_maxlevel);
if (level < bmp->db_maxlevel) {
assert(level == bmp->db_maxlevel);
dbmp_le->dn_maxlevel = cpu_to_le32(bmp->db_maxlevel);
bmp->db_maxlevel = BMAPSZTOLEV(bmp->db_mapsize);