workqueue: change BUG_ON() to WARN_ON()

This BUG_ON() can be triggered if you call schedule_work() before
calling INIT_WORK().  It is a bug definitely, but it's nicer to just
print a stack trace and return.
Reported-by: Matt Renzelmann <mjr@cs.wisc.edu>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/workqueue.c

@@ -1032,7 +1032,10 @@ static void __queue_work(unsigned int cpu, struct workqueue_struct *wq,
 	cwq = get_cwq(gcwq->cpu, wq);
 	trace_workqueue_queue_work(cpu, cwq, work);
 
-	BUG_ON(!list_empty(&work->entry));
+	if (WARN_ON(!list_empty(&work->entry))) {
+		spin_unlock_irqrestore(&gcwq->lock, flags);
+		return;
+	}
 
 	cwq->nr_in_flight[cwq->work_color]++;
 	work_flags = work_color_to_flags(cwq->work_color);