workqueue: devres: Add device-managed allocate workqueue

workqueue: devres: Add device-managed allocate workqueue

Add a Resource-managed version of alloc_workqueue() to fix common
problem of drivers mixing devm() calls with destroy_workqueue.  Such
naive and discouraged driver approach leads to difficult to debug bugs
when the driver:

1. Allocates workqueue in standard way and destroys it in driver
   remove() callback,
2. Sets work struct with devm_work_autocancel(),
3. Registers interrupt handler with devm_request_threaded_irq().

Which leads to following unbind/removal path:

1. destroy_workqueue() via driver remove(),
   Any interrupt coming now would still execute the interrupt handler,
   which queues work on destroyed workqueue.
2. devm_irq_release(),
3. devm_work_drop() -> cancel_work_sync() on destroyed workqueue.

devm_alloc_workqueue() has two benefits:
1. Solves above problem of mix-and-match devres and non-devres code in
   driver,
2. Simplify any sane drivers which were correctly using
   alloc_workqueue() + devm_add_action_or_reset().

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@oss.qualcomm.com>
Acked-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Tejun Heo <tj@kernel.org>