xen-netfront: cast grant table reference first to type int

IS_ERR_VALUE() in commit 87557efc
("xen-netfront: do not cast grant table reference to signed short") would
not return true for error code unless we cast ref first to type int.
Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/xen-netfront.c

@@ -304,7 +304,7 @@ static void xennet_alloc_rx_buffers(struct netfront_queue *queue)
 		queue->rx_skbs[id] = skb;
 
 		ref = gnttab_claim_grant_reference(&queue->gref_rx_head);
-		WARN_ON_ONCE(IS_ERR_VALUE((unsigned long)ref));
+		WARN_ON_ONCE(IS_ERR_VALUE((unsigned long)(int)ref));
 		queue->grant_rx_ref[id] = ref;
 
 		page = skb_frag_page(&skb_shinfo(skb)->frags[0]);
@@ -428,7 +428,7 @@ static void xennet_tx_setup_grant(unsigned long gfn, unsigned int offset,
 	id = get_id_from_freelist(&queue->tx_skb_freelist, queue->tx_skbs);
 	tx = RING_GET_REQUEST(&queue->tx, queue->tx.req_prod_pvt++);
 	ref = gnttab_claim_grant_reference(&queue->gref_tx_head);
-	WARN_ON_ONCE(IS_ERR_VALUE((unsigned long)ref));
+	WARN_ON_ONCE(IS_ERR_VALUE((unsigned long)(int)ref));
 
 	gnttab_grant_foreign_access_ref(ref, queue->info->xbdev->otherend_id,
 					gfn, GNTMAP_readonly);