time: don't touch an offlined CPU's ts->tick_stopped in tick_cancel_sched_timer()

Silences WARN_ONs in rcu_enter_nohz() and rcu_exit_nohz(), which appeared
before caused by (repeated) calls to:
        $ echo 0 > /sys/devices/system/cpu/cpu1/online
        $ echo 1 > /sys/devices/system/cpu/cpu1/online

Signed-off-by: Karsten Wiese <fzu@wemgehoertderstaat.de>
Cc: johnstul@us.ibm.com
Cc: Rafael Wysocki <rjw@sisk.pl>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Ingo Molnar <mingo@elte.hu>
Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

kernel/time/tick-sched.c

@@ -640,7 +640,7 @@ void tick_cancel_sched_timer(int cpu)
 
 	if (ts->sched_timer.base)
 		hrtimer_cancel(&ts->sched_timer);
-	ts->tick_stopped = 0;
+
 	ts->nohz_mode = NOHZ_MODE_INACTIVE;
 }
 #endif /* HIGH_RES_TIMERS */