set session_states may walk around the privilege check to set resource group

[djshow832]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

connection default;
CREATE USER no_resource_group;
CREATE RESOURCE GROUP test RU_PER_SEC = 666;
GRANT RESOURCE_GROUP_USER ON *.* TO resource_group_user;

connect (no_resource_group,localhost,no_resource_group,,);
SET SESSION_STATES "{'rs-group':'test'}";
SELECT CURRENT_RESOURCE_GROUP();

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

SELECT CURRENT_RESOURCE_GROUP();
default

3. What did you see instead (Required)

SELECT CURRENT_RESOURCE_GROUP();
test

4. What is your TiDB version? (Required)

master