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>

mm/mmu_context.c

@@ -31,6 +31,9 @@ void use_mm(struct mm_struct *mm)
 	tsk->mm = mm;
 	switch_mm(active_mm, mm, tsk);
 	task_unlock(tsk);
+#ifdef finish_arch_post_lock_switch
+	finish_arch_post_lock_switch();
+#endif
 
 	if (active_mm != mm)
 		mmdrop(active_mm);