tidb can point update data even if tidb_super_read_only is on

[ichn-hu]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

create table t (a int primary key, b int)
insert into t values (1, 2), (3, 4);
set global tidb_restricted_read_only=1;
update table t set b = 1 where a = 1;

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

ERROR 1836 (HY000): Running in read-only mode

3. What did you see instead (Required)

Query OK, 1 row affected (0.002 sec)
Rows matched: 1  Changed: 1  Warnings: 0

4. What is your TiDB version? (Required)

master