NILFS_BMAP_GROUP_DIV
BUILD_BUG_ON_NOT_POWER_OF_2(NILFS_BMAP_GROUP_DIV);
index = bmap->b_inode->i_ino & (NILFS_BMAP_GROUP_DIV - 1);
index * (entries_per_group / NILFS_BMAP_GROUP_DIV);