loop: zero-fill bio on the submitting cpu

In thruth I've just audited which blk-mq drivers don't currently have a
complete callback, but I think this change is at least borderline useful.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/drivers/block/loop.h b/drivers/block/loop.h
index fb2237c..fecd3f9 100644
--- a/drivers/block/loop.h
+++ b/drivers/block/loop.h
@@ -70,6 +70,7 @@ struct loop_cmd {
 	struct request *rq;
 	struct list_head list;
 	bool use_aio;           /* use AIO interface to handle I/O */
+	long ret;
 	struct kiocb iocb;
 };