sched: Clear idle_stamp when unthrottle offline tasks

hulk inclusion
category: bugfix
bugzilla: 51828, https://gitee.com/openeuler/kernel/issues/I4K96G


CVE: NA

--------------------------------

When unthrottle offline tasks successfully, we should clear
idle_stamp.

Signed-off-by: Zhang Qiao <zhangqiao22@huawei.com>
Signed-off-by: Zheng Zucheng <zhengzucheng@huawei.com>
Reviewed-by: Chen Hui <judy.chenhui@huawei.com>
Reviewed-by: Xiu Jianfeng <xiujianfeng@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>