sched_ext: Introduce scx_bpf_cgroup_nr_cpus()

Hierarchical BPF schedulers may need to know how many CPUs a cgroup can
run on when distributing group-wide weights.

For example, fair's group share calculation scales a group's weight by
the smaller of its estimated runnable task count and the number of CPUs
in its effective cpuset. Without that count, a BPF scheduler can only
use the machine-wide CPU count and may give a group confined to a few
CPUs excessive shares.

A task's effective affinity can include per-task restrictions, so it
cannot reliably represent the group's cpuset. Reading the cpuset
directly from BPF would require CO-RE access to private cgroup and
cpuset structures, including inherited cpuset resolution, making the
approach fragile across kernel versions.

Add scx_bpf_cgroup_nr_cpus(), which returns the same count fair uses
through cpuset_num_cpus(). The count is independent of CPU or CID
numbering, so the kfunc is available to both cpu-form and cid-form
schedulers from any context. Also add a compat wrapper which falls
back to nr_cpu_ids on older kernels.

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

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