Inaccurate size of raft entry commits failed after adjusting commit batch size

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

1. Prepare 1 million rows into sbtest table using sysbench
2. Set global tidb_txn_commit_batch_size to 2500000
3. Execute the following SQL
```
create table test like sbtest;
insert into test select * from sbtest;
```
### 2. What did you expect to see? (Required)

Insert statement success.

### 3. What did you see instead (Required)

ERROR 1105 (HY000): message:"raft entry is too large, region 96, entry size 8815878" raft_entry_too_large:

### 4. What is your TiDB version? (Required)
v6.5.2