[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>

net/sched/sch_ingress.c

@@ -362,12 +362,12 @@ static void ingress_destroy(struct Qdisc *sch)
 
 static int ingress_dump(struct Qdisc *sch, struct sk_buff *skb)
 {
-	unsigned char *b = skb->tail;
+	unsigned char *b = skb_tail_pointer(skb);
 	struct rtattr *rta;
 
 	rta = (struct rtattr *) b;
 	RTA_PUT(skb, TCA_OPTIONS, 0, NULL);
-	rta->rta_len = skb->tail - b;
+	rta->rta_len = skb_tail_pointer(skb) - b;
 	return skb->len;
 
 rtattr_failure: