sched_ext: Deliver ecaps only while the root has the cpu online

A sub-scheduler's ecaps ops run in dispatch context, and ops.cpu_online()
and ops.cpu_offline() document that the cpu doesn't call ops.dispatch()
outside them. The reseed of a cpu coming back is queued before the root's
ops.cpu_online(), so the ops of a sub-scheduler and its parent can run for a
cpu the root still considers offline.

Add SCX_RQ_OPS_ONLINE, set while the root considers the cpu online, from its
ops.cpu_online() or its enable until its ops.cpu_offline(). A sync outside
that span leaves the ecaps alone, and the reseed, now queued after
ops.cpu_online(), reports the net change. SCX_RQ_ONLINE can't be used, as
the core changes it ahead of the root's view on both edges.

A cpu going offline still drops its caps without a report.

While at it, fix the SCX_RQ_ONLINE comments, which claimed that the flag
tracks ops.cpu_online() and ops.cpu_offline().

v2: Fix the SCX_RQ_ONLINE comments.

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

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