space max size is not same with any mysql version

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)
execute this sql in mysql 5.6,mysql 5.7 mysql 8.0 and tidb 6.1
select length(space(16777216)) as a,length(space(16777218)) as b,length(space(2)) as c,version() as v;
in mysql 5.6
in mysql 5.7
in mysql 8.0
in tidb 6.1

### 2. What did you expect to see? (Required)
space max size is same with some mysql version
### 3. What did you see instead (Required)
space max size is not same with any mysql version
### 4. What is your TiDB version? (Required)

Release Version: v6.1.0
Edition: Community
Git Commit Hash: 1a89decdb192cbdce6a7b0020d71128bc964d30f
Git Branch: heads/refs/tags/v6.1.0
UTC Build Time: 2022-06-05 05:09:33
GoVersion: go1.18.2
Race Enabled: false
TiKV Min Version: v3.0.0-60965b006877ca7234adaced7890d7b029ed1306
Check Table Before Drop: false