sched/smp: Use the SMP version of is_migration_disabled()

Simplify the scheduler by making the CONFIG_SMP-only code in
is_migration_disabled() unconditional.

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/20250528080924.2273858-38-mingo@kernel.org

kernel/sched/sched.h

@@ -1301,11 +1301,7 @@ static inline int cpu_of(struct rq *rq)
 
 static inline bool is_migration_disabled(struct task_struct *p)
 {
-#ifdef CONFIG_SMP
 	return p->migration_disabled;
-#else
-	return false;
-#endif
 }
 
 DECLARE_PER_CPU_SHARED_ALIGNED(struct rq, runqueues);