sched_ext: Add per-shard scx_sched storage scaffolding

Add struct scx_pshard and sch->pshard[] indexed by shard_idx, each entry
allocated on its shard's NUMA node from scx_shard_node[si]. The struct
starts empty (one dummy field). Follow-up patches will grow it as
shard-local state lands. Only cid-type schedulers with an arena pool get
pshards.

Allocation happens after ops.init_cids() returns so any
scx_bpf_cid_override() it issues has finalized scx_nr_cid_shards and
scx_shard_node[]. sch->nr_pshards records the array size for the async RCU
free path, which may run after a later scheduler's scx_cid_init() has
rewritten the global.

v3: Build and publish pshard[] fully-formed here rather than a later patch.
v2: Free the partially-allocated pshard array on alloc failure. (sashiko AI)

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

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