udf_read_dscr_phys
error = udf_read_dscr_phys(lbnum, &dscr);
error = udf_read_dscr_phys(vat_loc, &vat_candidate);
error = udf_read_dscr_phys(lb_num, &dscr);
extern int udf_read_dscr_phys(uint32_t sector, union dscrptr **dstp);