sched/fair: Hoist idle_stamp up from idle_balance

maillist inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I8PIYZ
CVE: NA

Reference: https://lore.kernel.org/lkml/1541767840-93588-6-git-send-email-steven.sistare@oracle.com/



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

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 Yu <serein.chengyu@huawei.com>