xen-netback: avoid allocating variable size array on stack

Tune xen_netbk_count_requests to not touch working array beyond limit, so that
we can make working array size constant.
Suggested-by: NJan Beulich <jbeulich@suse.com>
Signed-off-by: NWei Liu <wei.liu2@citrix.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/xen-netback/netback.c

@@ -934,11 +934,14 @@ static int netbk_count_requests(struct xenvif *vif,
 	RING_IDX cons = vif->tx.req_cons;
 	int slots = 0;
 	int drop_err = 0;
+	int more_data;
 
 	if (!(first->flags & XEN_NETTXF_more_data))
 		return 0;
 
 	do {
+		struct xen_netif_tx_request dropped_tx = { 0 };
+
 		if (slots >= work_to_do) {
 			netdev_err(vif->dev,
 				   "Asked for %d slots but exceeds this limit\n",
@@ -972,6 +975,9 @@ static int netbk_count_requests(struct xenvif *vif,
 			drop_err = -E2BIG;
 		}
 
+		if (drop_err)
+			txp = &dropped_tx;
+
 		memcpy(txp, RING_GET_REQUEST(&vif->tx, cons + slots),
 		       sizeof(*txp));
 
@@ -1001,7 +1007,13 @@ static int netbk_count_requests(struct xenvif *vif,
 			netbk_fatal_tx_err(vif);
 			return -EINVAL;
 		}
-	} while ((txp++)->flags & XEN_NETTXF_more_data);
+
+		more_data = txp->flags & XEN_NETTXF_more_data;
+
+		if (!drop_err)
+			txp++;
+
+	} while (more_data);
 
 	if (drop_err) {
 		netbk_tx_err(vif, first, cons + slots);
@@ -1408,7 +1420,7 @@ static unsigned xen_netbk_tx_build_gops(struct xen_netbk *netbk)
 		!list_empty(&netbk->net_schedule_list)) {
 		struct xenvif *vif;
 		struct xen_netif_tx_request txreq;
-		struct xen_netif_tx_request txfrags[max_skb_slots];
+		struct xen_netif_tx_request txfrags[XEN_NETIF_NR_SLOTS_MIN];
 		struct page *page;
 		struct xen_netif_extra_info extras[XEN_NETIF_EXTRA_TYPE_MAX-1];
 		u16 pending_idx;