[NET]: Remove gratuitous use of skb->tail in network drivers.

Many drivers use skb->tail unnecessarily.

In these situations, the code roughly looks like:

	dev = dev_alloc_skb(...);

	[optional] skb_reserve(skb, ...);

	... skb->tail ...

But even if the skb_reserve() happens, skb->data equals
skb->tail.  So it doesn't make any sense to use anything
other than skb->data in these cases.

Another case was the s2io.c driver directly mucking with
the skb->data and skb->tail pointers.  It really just wanted
to do an skb_reserve(), so that's what the code was changed
to do instead.

Another reason I'm making this change as it allows some SKB
cleanups I have planned simpler to merge.  In those cleanups,
skb->head, skb->tail, and skb->end pointers are removed, and
replaced with skb->head_room and skb->tail_room integers.

Signed-off-by: David S. Miller <davem@davemloft.net>
Acked-by: Jeff Garzik <jgarzik@pobox.com>

drivers/net/epic100.c

@@ -1003,7 +1003,7 @@ static void epic_init_ring(struct net_device *dev)
 		skb->dev = dev;			/* Mark as being used by this device. */
 		skb_reserve(skb, 2);	/* 16 byte align the IP header. */
 		ep->rx_ring[i].bufaddr = pci_map_single(ep->pci_dev, 
-			skb->tail, ep->rx_buf_sz, PCI_DMA_FROMDEVICE);
+			skb->data, ep->rx_buf_sz, PCI_DMA_FROMDEVICE);
 		ep->rx_ring[i].rxstatus = cpu_to_le32(DescOwn);
 	}
 	ep->dirty_rx = (unsigned int)(i - RX_RING_SIZE);
@@ -1274,7 +1274,7 @@ static int epic_rx(struct net_device *dev, int budget)
 							    ep->rx_ring[entry].bufaddr,
 							    ep->rx_buf_sz,
 							    PCI_DMA_FROMDEVICE);
-				eth_copy_and_sum(skb, ep->rx_skbuff[entry]->tail, pkt_len, 0);
+				eth_copy_and_sum(skb, ep->rx_skbuff[entry]->data, pkt_len, 0);
 				skb_put(skb, pkt_len);
 				pci_dma_sync_single_for_device(ep->pci_dev,
 							       ep->rx_ring[entry].bufaddr,
@@ -1308,7 +1308,7 @@ static int epic_rx(struct net_device *dev, int budget)
 			skb->dev = dev;			/* Mark as being used by this device. */
 			skb_reserve(skb, 2);	/* Align IP on 16 byte boundaries */
 			ep->rx_ring[entry].bufaddr = pci_map_single(ep->pci_dev, 
-				skb->tail, ep->rx_buf_sz, PCI_DMA_FROMDEVICE);
+				skb->data, ep->rx_buf_sz, PCI_DMA_FROMDEVICE);
 			work_done++;
 		}
 		ep->rx_ring[entry].rxstatus = cpu_to_le32(DescOwn);