Root user lack rights to use `tidb_mdl_view`

[tangenta]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

mysql> select * from mysql.tidb_mdl_view;
ERROR 1356 (HY000): View 'mysql.tidb_mdl_view' references invalid table(s) or column(s) or function(s) or definer/invoker of view lack rights to use them

mysql> select host, user, select_priv, process_priv from mysql.user;
+------+------+-------------+--------------+
| host | user | select_priv | process_priv |
+------+------+-------------+--------------+
| %    | root | Y           | Y            |
+------+------+-------------+--------------+
1 row in set (0.00 sec)

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

No error

3. What did you see instead (Required)

ERROR 1356

4. What is your TiDB version? (Required)

4b91fee