[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/8139cp.c

@@ -596,7 +596,7 @@ rx_status_loop:
 
 		mapping =
 		cp->rx_skb[rx_tail].mapping =
-			pci_map_single(cp->pdev, new_skb->tail,
+			pci_map_single(cp->pdev, new_skb->data,
 				       buflen, PCI_DMA_FROMDEVICE);
 		cp->rx_skb[rx_tail].skb = new_skb;
 
@@ -1101,7 +1101,7 @@ static int cp_refill_rx (struct cp_private *cp)
 		skb_reserve(skb, RX_OFFSET);
 
 		cp->rx_skb[i].mapping = pci_map_single(cp->pdev,
-			skb->tail, cp->rx_buf_sz, PCI_DMA_FROMDEVICE);
+			skb->data, cp->rx_buf_sz, PCI_DMA_FROMDEVICE);
 		cp->rx_skb[i].skb = skb;
 
 		cp->rx_ring[i].opts2 = 0;