[PATCHSET] implement and use port_task

[Tejun Heo <htejun@gmail.com>]

Hello, Jeff and Albert.

This patchset implements port_task and replace pio_task and
packet_task with it.  port_task isn't very different from pio_task or
packet_task - it's straight forward generalization.  The port_task is
a per-port task avaliable to low level drivers which is managed by
libata core layer.

Differences from pio/packet task.

* Not limited to PIO HSM implementation.  Other drivers can use it.

* Synchronization against EH core layer is done within libata core
  layer, not in PIO HSM implementation (ata_qc_timeout()).

Thanks.

--
tejun