ext2_blkoff
off = ext2_blkoff(fs, fp->f_seekp);
offset = ext2_blkoff(fs, length);
blkoffset = ext2_blkoff(fs, uio->uio_offset);
blkoffset = ext2_blkoff(fs, uio->uio_offset);
blkoffset = ext2_blkoff(fs, uio->uio_offset);
*res = (char *)bp->b_data + ext2_blkoff(fs, offset);