unexpected admin checksum result in unistore

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

``` SQL
create table tb(id int primary key) partition by hash (id) partitions 16;
insert into tb values(1);
admin checksum table tb;
```

### 2. What did you expect to see? (Required)

``` SQL
mysql> admin checksum table tb;
+---------+------------+----------------------+-----------+-------------+
| Db_name | Table_name | Checksum_crc64_xor | Total_kvs | Total_bytes |
+---------+------------+----------------------+-----------+-------------+
| test | tb | 15431044026820383696 | 1 | 25 |
+---------+------------+----------------------+-----------+-------------+
1 row in set (0.01 sec)
```

### 3. What did you see instead (Required)
``` SQL
mysql> admin checksum table tb;
+---------+------------+--------------------+-----------+-------------+
| Db_name | Table_name | Checksum_crc64_xor | Total_kvs | Total_bytes |
+---------+------------+--------------------+-----------+-------------+
| test | tb | 1 | 17 | 17 |
+---------+------------+--------------------+-----------+-------------+
1 row in set (0.01 sec)

Total_kvs should be 1

```
### 4. What is your TiDB version? (Required)
master