blk-mq: remove the error argument to blk_mq_complete_request

[hch at lst.de]

On Tue, Apr 18, 2017 at 10:50:18PM +0000, Bart Van Assche wrote:
> On Tue, 2017-04-18 at 08:52 -0700, Christoph Hellwig wrote:
> > Now that we always have a ->complete callback we can remove the direct
> > call to blk_mq_end_request, as well as the error argument to
> > blk_mq_complete_request.
> 
> Hello Christoph,
> 
> Please add a runtime check that issues a warning early if a .complete
> callback function is missing.
> 
> Are you sure that all blk-mq drivers have a .complete callback? In the
> request-errors branch of your block git tree I found the following:

I think the problem is that my above changelog was wrong - we only
require the complete_rq method if the driver calls the
blk_mq_complete_request function.  Both rbd and ubiblock don't in
the tree.  They probably should, but that's work for a separate
series.