atl1: fix 4G memory corruption bug

When using 4+ GB RAM and SWIOTLB is active, the driver corrupts
memory by writing an skb after the relevant DMA page has been
unmapped.  Although this doesn't happen when *not* using bounce
buffers, clearing the pointer to the DMA page after unmapping
it fixes the problem.

http://marc.info/?t=120861317000005&r=2&w=2

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Jay Cliburn <jacliburn@bellsouth.net>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>

drivers/net/atlx/atl1.c

@@ -2023,6 +2023,7 @@ rrd_ok:
 		/* Good Receive */
 		pci_unmap_page(adapter->pdev, buffer_info->dma,
 			       buffer_info->length, PCI_DMA_FROMDEVICE);
+		buffer_info->dma = 0;
 		skb = buffer_info->skb;
 		length = le16_to_cpu(rrd->xsz.xsum_sz.pkt_size);