[S390] etr/stp: fix possible deadlock

Precreate stop_machine threads in case the machine supports ETR/STP.
Otherwise we might deadlock if a time sync operation gets scheduled
and the creation of stop_machine threads would cause disk I/O.
This is just the minimal fix.
The real fix would be to only precreate stop_machine threads if
ETR/STP is actually used. But that would be a rather large and
complicated patch.
Signed-off-by: NHeiko Carstens <heiko.carstens@de.ibm.com>

arch/s390/kernel/time.c

@@ -399,8 +399,10 @@ static struct workqueue_struct *time_sync_wq;
 
 static void __init time_init_wq(void)
 {
-	if (!time_sync_wq)
-		time_sync_wq = create_singlethread_workqueue("timesync");
+	if (time_sync_wq)
+		return;
+	time_sync_wq = create_singlethread_workqueue("timesync");
+	stop_machine_create();
 }
 
 /*