netxen: fix infinite loop on dma mapping failure

Fix a perpetual while() loop in unwinding partial
mapped tx skb on dma mapping failure.

Reported-by: "Juha Leppanen" <juha_motorsportcom@luukku.com>
Signed-off-by: Dhananjay Phadke <dhananjay@netxen.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/netxen/netxen_nic_main.c

@@ -1577,8 +1577,8 @@ netxen_map_tx_skb(struct pci_dev *pdev,
 	return 0;
 
 unwind:
-	while (i > 0) {
-		nf = &pbuf->frag_array[i];
+	while (--i >= 0) {
+		nf = &pbuf->frag_array[i+1];
 		pci_unmap_page(pdev, nf->dma, nf->length, PCI_DMA_TODEVICE);
 	}