sched/core: Avoid _cond_resched() for PREEMPT=y

On fully preemptible kernels _cond_resched() is pointless, so avoid
emitting any code for it.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mikulas Patocka <mpatocka@redhat.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/sched/core.c

@@ -4883,6 +4883,7 @@ SYSCALL_DEFINE0(sched_yield)
 	return 0;
 }
 
+#ifndef CONFIG_PREEMPT
 int __sched _cond_resched(void)
 {
 	if (should_resched(0)) {
@@ -4892,6 +4893,7 @@ int __sched _cond_resched(void)
 	return 0;
 }
 EXPORT_SYMBOL(_cond_resched);
+#endif
 
 /*
  * __cond_resched_lock() - if a reschedule is pending, drop the given lock,