sched_ext: Add coalescing sub_caps_updated() notifier for sub-schedulers

Wire up ops_cid.sub_caps_updated() to notify sub-scheds of cap changes.

Three constraints shape the design:

  1. Static memory. Deliveries use a fixed-size buffer, both for runtime
     efficiency and so notifications can't be lost under memory pressure.

  2. High-frequency updates. Grant/revoke can mutate caps in bursts, and the
     notifier path must absorb that without amplifying it.

  3. Recursive grant/revoke from the callback. A child receiving a
     notification can call grant/revoke on its own children, which can
     cascade recursively down its subtree.

(1) and (2) lead to coalescing into a fixed payload. Each delivery carries a
single (cmask, caps) pair covering every change since the previous one.
Direction (set vs cleared) isn't encoded as it doesn't fit in the fixed-size
summary. The callback queries scx_bpf_sub_caps() for current state. Only one
delivery is in flight per shard. Further changes fold into the same buffer
and ship as the next callback, so a shard's callbacks fire in order.

(3) leads to deferred delivery. Events accumulate during grant/revoke and
are delivered after the shard lock is released.

v2:
- Request a private stack for ops.sub_caps_updated(). (sashiko AI)
- Build cmask_arena_out via scx_cmask_ref, not by re-reading its header.

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

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