[SK_BUFF]: Convert skb->tail to sk_buff_data_t

So that it is also an offset from skb->head, reduces its size from 8 to 4 bytes
on 64bit architectures, allowing us to combine the 4 bytes hole left by the
layer headers conversion, reducing struct sk_buff size to 256 bytes, i.e. 4
64byte cachelines, and since the sk_buff slab cache is SLAB_HWCACHE_ALIGN...
:-)

Many calculations that previously required that skb->{transport,network,
mac}_header be first converted to a pointer now can be done directly, being
meaningful as offsets or pointers.

Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

arch/ia64/sn/kernel/xpnet.c

@@ -264,7 +264,7 @@ xpnet_receive(partid_t partid, int channel, struct xpnet_message *msg)
 
 	dev_dbg(xpnet, "<skb->head=0x%p skb->data=0x%p skb->tail=0x%p "
 		"skb->end=0x%p skb->len=%d\n", (void *) skb->head,
-		(void *) skb->data, (void *) skb->tail, (void *) skb->end,
+		(void *)skb->data, skb_tail_pointer(skb), (void *)skb->end,
 		skb->len);
 
 	skb->protocol = eth_type_trans(skb, xpnet_device);
@@ -272,7 +272,7 @@ xpnet_receive(partid_t partid, int channel, struct xpnet_message *msg)
 
 	dev_dbg(xpnet, "passing skb to network layer; \n\tskb->head=0x%p "
 		"skb->data=0x%p skb->tail=0x%p skb->end=0x%p skb->len=%d\n",
-		(void *) skb->head, (void *) skb->data, (void *) skb->tail,
+		(void *)skb->head, (void *)skb->data, skb_tail_pointer(skb),
 		(void *) skb->end, skb->len);
 
 
@@ -475,7 +475,7 @@ xpnet_dev_hard_start_xmit(struct sk_buff *skb, struct net_device *dev)
 
 	dev_dbg(xpnet, ">skb->head=0x%p skb->data=0x%p skb->tail=0x%p "
 		"skb->end=0x%p skb->len=%d\n", (void *) skb->head,
-		(void *) skb->data, (void *) skb->tail, (void *) skb->end,
+		(void *)skb->data, skb_tail_pointer(skb), (void *)skb->end,
 		skb->len);
 
 
@@ -497,7 +497,7 @@ xpnet_dev_hard_start_xmit(struct sk_buff *skb, struct net_device *dev)
 
 	/* get the beginning of the first cacheline and end of last */
 	start_addr = ((u64) skb->data & ~(L1_CACHE_BYTES - 1));
-	end_addr = L1_CACHE_ALIGN((u64) skb->tail);
+	end_addr = L1_CACHE_ALIGN((u64)skb_tail_pointer(skb));
 
 	/* calculate how many bytes to embed in the XPC message */
 	embedded_bytes = 0;
@@ -573,7 +573,7 @@ xpnet_dev_hard_start_xmit(struct sk_buff *skb, struct net_device *dev)
 		msg->magic = XPNET_MAGIC;
 		msg->size = end_addr - start_addr;
 		msg->leadin_ignore = (u64) skb->data - start_addr;
-		msg->tailout_ignore = end_addr - (u64) skb->tail;
+		msg->tailout_ignore = end_addr - (u64)skb_tail_pointer(skb);
 		msg->buf_pa = __pa(start_addr);
 
 		dev_dbg(xpnet, "sending XPC message to %d:%d\nmsg->buf_pa="