genirq/affinity: Only build SMP-only helper functions on SMP kernels

allnoconfig grew these new build warnings in lib/group_cpus.c:

  lib/group_cpus.c:247:12: warning: ‘__group_cpus_evenly’ defined but not used [-Wunused-function]
  lib/group_cpus.c:75:13: warning: ‘build_node_to_cpumask’ defined but not used [-Wunused-function]
  lib/group_cpus.c:66:13: warning: ‘free_node_to_cpumask’ defined but not used [-Wunused-function]
  lib/group_cpus.c:43:23: warning: ‘alloc_node_to_cpumask’ defined but not used [-Wunused-function]

Widen the #ifdef CONFIG_SMP block to not expose unused helpers on
non-SMP builds.

Also annotate the preprocessor branches for better readability.

Fixes: f7b3ea8c ("genirq/affinity: Move group_cpus_evenly() into lib/")
Cc: Ming Lei <ming.lei@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20221227022905.352674-6-ming.lei@redhat.com


Signed-off-by: Ingo Molnar <mingo@kernel.org>