dm mpath: merge do_end_io_bio into multipath_end_io_bio

This simplifies the code and especially the error passing a bit and
will help with the next patch.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Jens Axboe <axboe@fb.com>