!6420 tty: fix read of tty->pgrp outside of ctrl_lock

Merge Pull Request from: @ci-robot 
 
PR sync from: Yi Yang <yiyang13@huawei.com>
https://mailweb.openeuler.org/hyperkitty/list/kernel@openeuler.org/message/AS7N5YHGYLF2CH3KHYDOHKM7X6VERS5O/ 
 
https://gitee.com/openeuler/kernel/issues/I9IPML 
 
Link:https://gitee.com/openeuler/kernel/pulls/6420

 

Reviewed-by: Liu YongQiang <liuyongqiang13@huawei.com>
Signed-off-by: Zhang Changzhong <zhangchangzhong@huawei.com>