ixgb: fix bug when freeing resources

It was pointed out by Breno Leitao <[e-mail]> that
ixgb would crash on PPC when an IOMMU was in use, if change_mtu was
called.

It appears to be a pretty simple issue in the driver that wasn't discovered
because most systems don't run with an IOMMU. The driver needs to only unmap
buffers that are mapped (duh).

CC: Breno Leitao <[e-mail]>

Signed-off-by: Jesse Brandeburg <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/ixgb/ixgb_main.c

···
 
 	for (i = 0; i < rx_ring->count; i++) {
 		buffer_info = &rx_ring->buffer_info[i];
-		if (buffer_info->skb) {
-
+		if (buffer_info->dma) {
 			pci_unmap_single(pdev,
 					 buffer_info->dma,
 					 buffer_info->length,
 					 PCI_DMA_FROMDEVICE);
+			buffer_info->dma = 0;
+			buffer_info->length = 0;
+		}
 
+		if (buffer_info->skb) {
 			dev_kfree_skb(buffer_info->skb);
-
 			buffer_info->skb = NULL;
 		}
 	}