workqueue: fix devm_alloc_workqueue() va_list misuse

workqueue: fix devm_alloc_workqueue() va_list misuse

devm_alloc_workqueue() built a va_list and passed it as a single
positional argument to the variadic alloc_workqueue() macro:

	va_start(args, max_active);
	wq = alloc_workqueue(fmt, flags, max_active, args);
	va_end(args);

C does not allow forwarding a va_list through a ... parameter.
alloc_workqueue() expands to alloc_workqueue_noprof(), which runs
its own va_start() over its ... params, so the inner
vsnprintf(wq->name, sizeof(wq->name), fmt, args) in
__alloc_workqueue() received the outer va_list object as the first
variadic slot rather than the caller's actual format arguments.

Add a new static helper alloc_workqueue_va() that wraps
__alloc_workqueue() and runs wq_init_lockdep() on success, and
fold both alloc_workqueue_noprof() and devm_alloc_workqueue_noprof()
onto it as suggested by Tejun.

The wq_init_lockdep() step is required on the devm path
too, otherwise __flush_workqueue()'s on-stack
COMPLETION_INITIALIZER_ONSTACK_MAP would NULL-deref wq->lockdep_map.

No caller changes are required. devm_alloc_ordered_workqueue() is
a macro forwarding to devm_alloc_workqueue() and inherits the fix.
Two in-tree callers actively trigger the broken path on every probe:

  drivers/power/supply/mt6370-charger.c:889
  drivers/power/supply/max77705_charger.c:649

both of which use devm_alloc_ordered_workqueue(dev, "%s", 0,
dev_name(dev)).

A standalone reproducer module is available at[1].

Link: https://github.com/leitao/debug/blob/main/workqueue/valist/wq_va_test.c
Fixes: 1dfc9d60a69e ("workqueue: devres: Add device-managed allocate workqueue")
Signed-off-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Tejun Heo <tj@kernel.org>