virtio-blk: modify the value type of num in virtio_queue_rq()

This was found by coccicheck:
./drivers/block/virtio_blk.c, 334, 14-17, WARNING Unsigned expression
compared with zero  num < 0

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Ye Guojin <ye.guojin@zte.com.cn>
Link: https://lore.kernel.org/r/20211117063955.160777-1-ye.guojin@zte.com.cn
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Fixes: 02746e26c39e ("virtio-blk: avoid preallocating big SGL for data")
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Max Gurtovoy <mgurtovoy@nvidia.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
index 1a1b118..6ae3877 100644
--- a/drivers/block/virtio_blk.c
+++ b/drivers/block/virtio_blk.c
@@ -316,7 +316,7 @@
 	struct request *req = bd->rq;
 	struct virtblk_req *vbr = blk_mq_rq_to_pdu(req);
 	unsigned long flags;
-	unsigned int num;
+	int num;
 	int qid = hctx->queue_num;
 	bool notify = false;
 	blk_status_t status;