infoschema.user_privileges privilege requirements is not dependent on user's privileges

[morgo]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

mysql> select current_user();
+----------------+
| current_user() |
+----------------+
| USAGE_USER@%   |
+----------------+
1 row in set (0.01 sec)

mysql> show grants for USAGE_USER;
+----------------------------------------+
| Grants for USAGE_USER@%                |
+----------------------------------------+
| GRANT USAGE ON *.* TO 'USAGE_USER'@'%' |
+----------------------------------------+
1 row in set (0.00 sec)

mysql> select * from information_schema.USER_PRIVILEGES limit 5;
+------------------+---------------+----------------+--------------+
| GRANTEE          | TABLE_CATALOG | PRIVILEGE_TYPE | IS_GRANTABLE |
+------------------+---------------+----------------+--------------+
| 'USAGE_USER'@'%' | def           | USAGE          | NO           |
| 'cloudAdmin'@'%' | def           | Process        | NO           |
| 'cloudAdmin'@'%' | def           | SHUTDOWN       | NO           |
| 'cloudAdmin'@'%' | def           | RELOAD         | NO           |
| 'root'@'%'       | def           | Select         | YES          |
+------------------+---------------+----------------+--------------+
5 rows in set (0.00 sec)

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

The usage user should only see their own access.

3. What did you see instead (Required)

Can see all users.

4. What is your TiDB version? (Required)

mysql> select tidb_version()\G
*************************** 1. row ***************************
tidb_version(): Release Version: v5.2.0-alpha-237-g560a3f7b2-dirty
Edition: Community
Git Commit Hash: 560a3f7b242df467b18cb9cd81b898a8be262744
Git Branch: master
UTC Build Time: 2021-07-08 16:42:26
GoVersion: go1.16.4
Race Enabled: false
TiKV Min Version: v3.0.0-60965b006877ca7234adaced7890d7b029ed1306
Check Table Before Drop: false
1 row in set (0.00 sec)

[morgo]

This is forked from #26062

[djshow832]

Fixed by #26070

[ti-srebot]

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