udf_readatoffset
int udf_readatoffset(struct unode *, int *, off_t, struct buf **,
error = udf_readatoffset(ump->um_vat, &size, *sector << 2, &bp, &data);
error = udf_readatoffset(up, &size, offset, &bp, &data);
error = udf_readatoffset(ds->node, &ds->size, ds->offset,
error = udf_readatoffset(ds->node, &ds->size, ds->offset,