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: f7b3ea8cf7 ("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>

lib/group_cpus.c

@@ -9,6 +9,8 @@
 #include <linux/sort.h>
 #include <linux/group_cpus.h>
 
+#ifdef CONFIG_SMP
+
 static void grp_spread_init_one(struct cpumask *irqmsk, struct cpumask *nmsk,
 				unsigned int cpus_per_grp)
 {
@@ -327,7 +329,6 @@ static int __group_cpus_evenly(unsigned int startgrp, unsigned int numgrps,
 	return done;
 }
 
-#ifdef CONFIG_SMP
 /**
  * group_cpus_evenly - Group all CPUs evenly per NUMA/CPU locality
  * @numgrps: number of groups
@@ -412,7 +413,7 @@ struct cpumask *group_cpus_evenly(unsigned int numgrps)
 	}
 	return masks;
 }
-#else
+#else /* CONFIG_SMP */
 struct cpumask *group_cpus_evenly(unsigned int numgrps)
 {
 	struct cpumask *masks = kcalloc(numgrps, sizeof(*masks), GFP_KERNEL);
@@ -424,4 +425,4 @@ struct cpumask *group_cpus_evenly(unsigned int numgrps)
 	cpumask_copy(&masks[0], cpu_possible_mask);
 	return masks;
 }
-#endif
+#endif /* CONFIG_SMP */