workqueue: wq_watchdog_touch is always called with valid CPU

Warn in the case it is called with cpu == -1. This does not appear
to happen anywhere.
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/workqueue.c

@@ -7526,6 +7526,8 @@ notrace void wq_watchdog_touch(int cpu)
 {
 	if (cpu >= 0)
 		per_cpu(wq_watchdog_touched_cpu, cpu) = jiffies;
+	else
+		WARN_ONCE(1, "%s should be called with valid CPU", __func__);
 
 	wq_watchdog_touched = jiffies;
 }