sched/fair: Hoist idle_stamp up from idle_balance

hulk inclusion
category: feature
bugzilla: 38261, https://bugzilla.openeuler.org/show_bug.cgi?id=23


CVE: NA

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Move the update of idle_stamp from idle_balance to the call site in
pick_next_task_fair, to prepare for a future patch that adds work to
pick_next_task_fair which must be included in the idle_stamp interval.
No functional change.

Signed-off-by: Steve Sistare <steven.sistare@oracle.com>
Signed-off-by: Cheng Jian <cj.chengjian@huawei.com>
Reviewed-by: Hanjun Guo <guohanjun@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Reviewed-by: Xie XiuQi <xiexiuqi@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>