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.