sched/fair: Make SCHED_IDLE entity be preempted in strict hierarchy

Consider the following cgroup:

                       root
                        |
             ------------------------
             |                      |
       normal_cgroup            idle_cgroup
             |                      |
   SCHED_IDLE task_A           SCHED_NORMAL task_B

According to the cgroup hierarchy, A should preempt B. But current
check_preempt_wakeup_fair() treats cgroup se and task separately, so B
will preempt A unexpectedly.
Unify the wakeup logic by {c,p}se_is_idle only. This makes SCHED_IDLE of
a task a relative policy that is effective only within its own cgroup,
similar to the behavior of NICE.

Also fix se_is_idle() definition when !CONFIG_FAIR_GROUP_SCHED.

Fixes: 30400039 ("sched: Cgroup SCHED_IDLE support")
Signed-off-by: Tianchen Ding <dtcccc@linux.alibaba.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Josh Don <joshdon@google.com>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Link: https://lkml.kernel.org/r/20240626023505.1332596-1-dtcccc@linux.alibaba.com