fat_bmap
err = fat_bmap(dir, iblock, &phys, &mapped_blocks, 0, false);
extern int fat_bmap(struct inode *inode, sector_t sector, sector_t *phys,
err = fat_bmap(inode, iblock, &phys, &mapped_blocks, create, false);
err = fat_bmap(inode, iblock, &phys, &mapped_blocks, create, false);
err = fat_bmap(inode, iblock, &bmap, &mapped_blocks, create, true);