dm mpath: don't check for req->errors

We'll get all proper errors reported through ->end_io and ->errors will
go away soon.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Jens Axboe <axboe@fb.com>