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: 02746e2 ("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>

Author: Ye Guojin

drivers/block/virtio_blk.c

@@ -316,7 +316,7 @@ static blk_status_t virtio_queue_rq(struct blk_mq_hw_ctx *hctx,
 	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;