sched/mm: call finish_arch_post_lock_switch in idle_task_exit and use_mm

The finish_arch_post_lock_switch is called at the end of the task
switch after all locks have been released. In concept it is paired
with the switch_mm function, but the current code only does the
call in finish_task_switch. Add the call to idle_task_exit and
use_mm. One use case for the additional calls is s390 which will
use finish_arch_post_lock_switch to wait for the completion of
TLB flush operations.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

kernel/sched/core.c

@@ -4692,8 +4692,10 @@ void idle_task_exit(void)
 
 	BUG_ON(cpu_online(smp_processor_id()));
 
-	if (mm != &init_mm)
+	if (mm != &init_mm) {
 		switch_mm(mm, &init_mm, current);
+		finish_arch_post_lock_switch();
+	}
 	mmdrop(mm);
 }