kernel/workqueue: Use __printf markup to silence compiler in function 'alloc_workqueue'

Silence warnings (triggered at W=1) by adding relevant __printf attributes.

  kernel/workqueue.c:4249:2: warning: function 'alloc_workqueue' might be a candidate for 'gnu_printf' format attribute [-Wsuggest-attribute=format]

Signed-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Tejun Heo <tj@kernel.org>