tools/sched_ext: scx_pair: fix pair_ctx indexing for CPU pairs

tools/sched_ext: scx_pair: fix pair_ctx indexing for CPU pairs

scx_pair sizes pair_ctx to nr_cpu_ids / 2, so valid pair_ctx keys are
dense pair indexes in the range [0, nr_cpu_ids / 2).

However, the userspace setup code stores pair_id as the first CPU number
in each pair. On an 8-CPU system with "-S 1", that produces pair IDs
0, 2, 4 and 6 for pairs [0,1], [2,3], [4,5] and [6,7]. CPUs in the
latter half then look up pair_ctx with out-of-range keys and the BPF
scheduler aborts with:

EXIT: scx_bpf_error (scx_pair.bpf.c:328: failed to lookup pairc and
in_pair_mask for cpu[5])

Assign pair_id using a dense pair counter instead so that each CPU pair
maps to a valid pair_ctx entry. Besides, reject odd CPU configuration, as
scx_pair requires all CPUs to be paired.

Fixes: f0262b102c7c ("tools/sched_ext: add scx_pair scheduler")
Signed-off-by: Zhao Mengmeng <zhaomengmeng@kylinos.cn>
Signed-off-by: Tejun Heo <tj@kernel.org>