libata: switch pio task from workqueue to slow-work

libata: switch pio task from workqueue to slow-work

A workqueue isn't a good fit for the pio task:

- It does not require per-CPU support, thus wasting many threads.
- The pio task would like to have more than one thread per CPU
  in some cases, for the single CPU case of having more than one
  pio device active.

So convert to slow-work instead, this is now possible with support
for delayed slow work and cancellation.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>