sched: Rely on synchronize_rcu_mult() de-duplication

The synchronize_rcu_mult() function now detects duplicate requests
for the same grace-period flavor and waits only once for each flavor.
This commit therefore removes the ugly #ifdef from sched_cpu_deactivate()
because synchronize_rcu_mult(call_rcu, call_rcu_sched) now does what
the #ifdef used to be needed for.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>

kernel/sched/core.c

@@ -5874,15 +5874,9 @@ int sched_cpu_deactivate(unsigned int cpu)
 	 * users of this state to go away such that all new such users will
 	 * observe it.
 	 *
-	 * For CONFIG_PREEMPT we have preemptible RCU and its sync_rcu() might
-	 * not imply sync_sched(), so wait for both.
-	 *
 	 * Do sync before park smpboot threads to take care the rcu boost case.
 	 */
-	if (IS_ENABLED(CONFIG_PREEMPT))
-		synchronize_rcu_mult(call_rcu, call_rcu_sched);
-	else
-		synchronize_rcu();
+	synchronize_rcu_mult(call_rcu, call_rcu_sched);
 
 	if (!sched_smp_initialized)
 		return 0;