sched_ext: Route ops.update_idle() to sub-schedulers and re-notify owed scheds

__scx_update_idle() notified only the root scheduler. A sub-scheduler that
holds a cid needs that cid's idle state to place and kick on it.

Deliver ops.update_idle() to every scheduler that holds SCX_CAP_BASE on the
transitioning cid. The root holds every cap, so a real transition always
reaches it.

Real transitions are not enough on their own. A cid that is already idle
when a sub-sched gains baseline access produces no transition, so the new
holder would never learn it is idle. The ecaps sync arms a re-notify on the
gain, and the next idle pick delivers ops.update_idle() to just that sched,
leaving holders that already track the cpu untouched. A matching loss of
baseline access drops any pending re-notify.

Bypass suppresses ops.update_idle() too, so a cpu that goes idle during a
bypass window and stays idle yields no transition to re-deliver on
un-bypass. Arm the same re-notify for every sched leaving bypass. The acute
case is a child granted cids during its own ops.sub_attach(). The grant
lands while the child is bypassed and the notify walk skips it, so on
un-bypass it holds cids it never saw go idle. The root is owed the same and
is armed through a separate per-rq flag, which keeps this working when
sub-schedulers are compiled out.

v2: Gate the idle catch-up in pick_task_idle() to avoid a double ops.update_idle(). (sashiko AI)

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Andrea Righi <arighi@nvidia.com>

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