ps3disk: dequeue in-flight request

Other than in issue error paths, ps3disk always completely finishes
fetched requests.  With full completion on error paths, it can be
easily converted to dequeueing model.

* After L1 r/w call failure, ps3disk_submit_request_sg() now fails the
  whole request.  Issue failure isn't likely to benefit from partial
  retry anyway and ps3disk uses full failure in completion error path
  too, so I don't think this amounts to any meaningful functionality
  loss.

* flush completion is converted to _all for consistency.  It doesn't
  make any functional difference.

[ Impact: dequeue in-flight request ]

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Geert Uytterhoeven <Geert.Uytterhoeven@sonycom.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>