which does not account for fragmented multiblock dir2.
ndbno was blindly being advanced by m_dirblkfsbs, but then
blkmap_next_off() would return the logical block of the next
mapped extent in blkmap, which may be within the current
(fragmented) dir2 multi-block, not the next multi-block,
because the extent index t hadn't been advanced.
Fix this by calling blkmap_next_off() until ndbno has advanced
into the next multiblock dir2 block, thereby keeping
the extent index t straight while properly advancing