rounddown_64
new_end = rounddown_64(irec->br_startoff + irec->br_blockcount,
return rounddown_64(off, mp->m_sb.sb_rextsize);
start = rounddown_64(offset, rounding);
offset = rounddown_64(offset, rounding);
blen = rounddown_64(blen, alloc_unit);
blen = rounddown_64(blen, alloc_unit);