virtio_blk: Fix signedness bug in virtblk_prep_rq()

The virtblk_map_data() function returns negative error codes, however, the
'nents' field of vbr->sg_table is an unsigned int, which causes the error
handling not to work correctly.

Cc: stable@vger.kernel.org
Fixes: 0e9911fa ("virtio-blk: support mq_ops->queue_rqs()")
Signed-off-by: Rafael Mendonca <rafaelmendsr@gmail.com>
Message-Id: <20221021204126.927603-1-rafaelmendsr@gmail.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Suwan Kim <suwan.kim027@gmail.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>

drivers/block/virtio_blk.c

@@ -334,14 +334,16 @@ static blk_status_t virtblk_prep_rq(struct blk_mq_hw_ctx *hctx,
 					struct virtblk_req *vbr)
 {
 	blk_status_t status;
+	int num;
 
 	status = virtblk_setup_cmd(vblk->vdev, req, vbr);
 	if (unlikely(status))
 		return status;
 
-	vbr->sg_table.nents = virtblk_map_data(hctx, req, vbr);
-	if (unlikely(vbr->sg_table.nents < 0))
+	num = virtblk_map_data(hctx, req, vbr);
+	if (unlikely(num < 0))
 		return virtblk_fail_to_queue(req, -ENOMEM);
+	vbr->sg_table.nents = num;
 
 	blk_mq_start_request(req);