workqueue: Annotate alloc_workqueue_va() with __printf(1, 0)

alloc_workqueue_va() forwards its va_list to __alloc_workqueue() which
ultimately feeds vsnprintf(). __alloc_workqueue() already carries
__printf(1, 0); the new wrapper needs the same annotation so format
string checking propagates through the forwarding.

Fixes: 0de4cb473a ("workqueue: fix devm_alloc_workqueue() va_list misuse")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202604300347.2LgXyteh-lkp@intel.com/
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/workqueue.c

@@ -5906,6 +5906,7 @@ static struct workqueue_struct *__alloc_workqueue(const char *fmt,
 	return NULL;
 }
 
+__printf(1, 0)
 static struct workqueue_struct *alloc_workqueue_va(const char *fmt,
 						   unsigned int flags,
 						   int max_active,