major : Fixed computation of index block size (Anton Altaparmakov)

major : Fixed computation of index block size (Anton Altaparmakov)

When the cluster size is bigger than the index block size, the index
block size unit is 512 (not the sector size) instead of the cluster size.
The partitions formatted by mkntfs and used by ntfs-3g were not
interoperable with Windows when the cluster size is bigger than 4K
and the sector size is not 512.