sched_ext: Report the ecaps loss when a cpu goes offline

When a cpu goes offline, a sub-scheduler's effective caps on its cid are
cleared, but neither the sub-scheduler nor its parent is notified. The
sub-scheduler keeps treating the cid as usable, and the parent can't act on
the loss, e.g. reset the cpuperf target.

Report the loss before the root's ops.cpu_offline() and the regain after its
ops.cpu_online(). The zeroing at offline was the last change to a sched's
ecaps outside a delivery, so they now change only with one. Syncs on an
inactive cpu are processed instead of discarded, and the reseed reschedules
the cpu itself, as they got no resched when queued. Suspend and hibernation
now report their cpu downs and ups as well. This is temporary, and a later
patch leaves them out.

Two things need care. A cpu going down marks a sync for each sub-scheduler,
and its hotplug thread waits until each marked sync has been processed. The
cpu's own ext picks process them, and the ext server is started in case a
fair task holds the cpu. Only the marked syncs count, so a flood of changes
on the cid can't hold up the hotplug.

The detach report reaches only active cpus. It holds off cpu hotplug, so a
cpu it skips has reported its loss as it went down, and no unreported loss
is left to drop.

Drop the cpuperf reset from scx_qmap's ops.sub_detach(), as its
ops.sub_child_ecaps_updated() now resets the targets on the loss reports.

v2: Reschedule from the irq work and document the regain order.

v3: Pack nr_ecaps_offline with kick_sync_pending to avoid a hole (Andrea).

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

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