workqueue: annotate alloc_workqueue() as printf

This also enables checking of allows alloc_ordered_workqueue().
Signed-off-by: Rolf Eike Beer <eb@emlix.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

include/linux/workqueue.h

@@ -399,9 +399,8 @@ extern struct workqueue_struct *system_freezable_power_efficient_wq;
  * RETURNS:
  * Pointer to the allocated workqueue on success, %NULL on failure.
  */
-struct workqueue_struct *alloc_workqueue(const char *fmt,
-					 unsigned int flags,
-					 int max_active, ...);
+__printf(1, 4) struct workqueue_struct *
+alloc_workqueue(const char *fmt, unsigned int flags, int max_active, ...);
 
 /**
  * alloc_ordered_workqueue - allocate an ordered workqueue