sched_ext: idle: Introduce node-aware idle cpu kfunc helpers

Introduce a new kfunc to retrieve the node associated to a CPU:

 int scx_bpf_cpu_node(s32 cpu)

Add the following kfuncs to provide BPF schedulers direct access to
per-node idle cpumasks information:

 const struct cpumask *scx_bpf_get_idle_cpumask_node(int node)
 const struct cpumask *scx_bpf_get_idle_smtmask_node(int node)
 s32 scx_bpf_pick_idle_cpu_node(const cpumask_t *cpus_allowed,
 				int node, u64 flags)
 s32 scx_bpf_pick_any_cpu_node(const cpumask_t *cpus_allowed,
 			       int node, u64 flags)

Moreover, trigger an scx error when any of the non-node aware idle CPU
kfuncs are used when SCX_OPS_BUILTIN_IDLE_PER_NODE is enabled.

Cc: Yury Norov [NVIDIA] <yury.norov@gmail.com>
Signed-off-by: Andrea Righi <arighi@nvidia.com>
Reviewed-by: Yury Norov [NVIDIA] <yury.norov@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>