btt_map_read
ret = btt_map_read(arena, premap, &postmap, &t_flag, &e_flag,
ret = btt_map_read(arena, premap, &new_map, &new_t,
ret = btt_map_read(arena, premap, &old_postmap, NULL, &e_flag,
ret = btt_map_read(arena, le32_to_cpu(log_new.lba), &map_entry,