Some admin command can be executed without super privilege

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

```sql
mysql -utest -h127.0.0.1 -P4000 -p
Enter password:
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 1350565896
Server version: 8.0.11-TiDB-v8.0.0-alpha-253-gd96244d34b TiDB Server (Apache License 2.0) Community Edition, MySQL 8.0 compatible

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Oracle is a registered trademark of Oracle Corporation and/or its
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owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql> admin reload expr_pushdown_blacklist;
Query OK, 0 rows affected (0.00 sec)

mysql> admin plugins enable p;
ERROR 1105 (HY000): plugin 'p' not found
-- plugin should exist similar bug

mysql> ADMIN FLUSH bindings;
Query OK, 0 rows affected (0.00 sec)

mysql> ADMIN CAPTURE bindings;
Query OK, 0 rows affected (0.01 sec)

mysql> ADMIN RELOAD bindings;
Query OK, 0 rows affected (0.00 sec)

mysql> show plugins;
Empty set (0.00 sec)

mysql> SHOW GRANTS FOR 'test'@'%';
+----------------------------------+
| Grants for test@% |
+----------------------------------+
| GRANT USAGE ON *.* TO 'test'@'%' |
+----------------------------------+
1 row in set (0.00 sec)
```

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

```sql
Error 8121 (HY000): privilege check for 'Super' fail
```

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

Executed successfully

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

master