sched/smp: Use the SMP version of the idle scheduling class

Simplify the scheduler by making CONFIG_SMP=y code in the
idle scheduling classunconditional.

Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Shrikanth Hegde <sshegde@linux.ibm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Valentin Schneider <vschneid@redhat.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Link: https://lore.kernel.org/r/20250528080924.2273858-35-mingo@kernel.org

kernel/sched/idle.c

@@ -432,7 +432,6 @@ void cpu_startup_entry(enum cpuhp_state state)
  * idle-task scheduling class.
  */
 
-#ifdef CONFIG_SMP
 static int
 select_task_rq_idle(struct task_struct *p, int cpu, int flags)
 {
@@ -444,7 +443,6 @@ balance_idle(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
 {
 	return WARN_ON_ONCE(1);
 }
-#endif /* CONFIG_SMP */
 
 /*
  * Idle tasks are unconditionally rescheduled:
@@ -531,11 +529,9 @@ DEFINE_SCHED_CLASS(idle) = {
 	.put_prev_task		= put_prev_task_idle,
 	.set_next_task          = set_next_task_idle,
 
-#ifdef CONFIG_SMP
 	.balance		= balance_idle,
 	.select_task_rq		= select_task_rq_idle,
 	.set_cpus_allowed	= set_cpus_allowed_common,
-#endif
 
 	.task_tick		= task_tick_idle,