sched_ext: Add sub_ecaps_updated() effective-cap change notifier

A sub-scheduler that gains or loses effective caps on a cpu may want to act
on it right away - e.g. place or preempt on a newly usable cpu. The existing
ops.sub_caps_updated() doesn't fit as it is delivered asynchronously to
scheduling operations and can arrive before the per-cpu effective caps go
live.

Add ops.sub_ecaps_updated(cid, before, after), a cid-form callback fired
from scx_process_sync_ecaps() when a sub-sched's effective caps on a cid
change. It runs in dispatch context so the sched can insert, kick or preempt
on the cid directly. @before is the caps as of the last delivery.

Cpu hotplug rides the same machinery. Going down zeroes each sched's ecaps
on the cpu's cid, with queued syncs discarded at consumption while the cpu
is inactive. Coming back up queues a sync for every sched. reported_ecaps is
kept across the down/up cycle, so the resync fires the callback only if
ownership actually changed while the cpu was down.

v2: Compute cid below the active-cpu guard; discard queued syncs on !cpu_active(). (sashiko AI)

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

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