sched_ext: Add bpf_sched_ext_ops_cid struct_ops type

sched_ext: Add bpf_sched_ext_ops_cid struct_ops type

cpumask is awkward from BPF and unusable from arena; cid/cmask work in
both. Sub-sched enqueue will need cmask. Without a full cid interface,
schedulers end up mixing forms - a subtle-bug factory.

Add sched_ext_ops_cid, which mirrors sched_ext_ops with cid/cmask
replacing cpu/cpumask in the topology-carrying callbacks.
cpu_acquire/cpu_release are deprecated and absent; a prior patch
moved them past @priv so the cid-form can omit them without
disturbing shared-field offsets.

The two structs share byte-identical layout up to @priv, so the
existing bpf_scx init/check hooks, has_op bitmap, and
scx_kf_allow_flags[] are offset-indexed and apply to both.
BUILD_BUG_ON in scx_init() pins the shared-field and renamed-callback
offsets so any future drift trips at boot.

The kernel<->BPF boundary translates between cpu and cid:

- A static key, enabled on cid-form sched load, gates the translation
  so cpu-form schedulers pay nothing.
- dispatch, update_idle, cpu_online/offline and dump_cpu translate
  the cpu arg at the callsite.
- select_cpu also translates the returned cid back to a cpu.
- set_cpumask is wrapped to synthesize a cmask in a per-cpu scratch
  before calling the cid-form callback.

All scheds in a hierarchy share one form. The static key drives the
hot-path branch.

v2: Use struct_size() for the set_cmask_scratch percpu alloc. Move
    cid-shard fields and assertions into the later cid-shard patch.

v3: Drop `static` on scx_set_cmask_scratch; add extern in ext_internal.h.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Cheng-Yang Chou <yphbchou0911@gmail.com>
Reviewed-by: Changwoo Min <changwoo@igalia.com>
Reviewed-by: Andrea Righi <arighi@nvidia.com>