s390/cmm: avoid add_timer on concurrently used timer

cmm_set_timer could be called concurrently from cmm_thread, cmm proc
handler, upon cmm smsg receive and timer function itself. To avoid
potential race condition and hitting BUG_ON in add_timer on already
pending timer simply reuse mod_timer which is according to
documentation "the only safe way to modify the timeout" with multiple
unserialized concurrent users. mod_timer can handle both active and
inactive timers which allows to carry out minor code simplification as
well.
Reviewed-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/mm/cmm.c

@@ -191,12 +191,7 @@ static void cmm_set_timer(void)
 			del_timer(&cmm_timer);
 		return;
 	}
-	if (timer_pending(&cmm_timer)) {
-		if (mod_timer(&cmm_timer, jiffies + cmm_timeout_seconds*HZ))
-			return;
-	}
-	cmm_timer.expires = jiffies + cmm_timeout_seconds*HZ;
-	add_timer(&cmm_timer);
+	mod_timer(&cmm_timer, jiffies + cmm_timeout_seconds * HZ);
 }
 
 static void cmm_timer_fn(struct timer_list *unused)