[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>
diff --git a/net/sched/sch_prio.c b/net/sched/sch_prio.c
index de889f2..5b37110 100644
--- a/net/sched/sch_prio.c
+++ b/net/sched/sch_prio.c
@@ -271,7 +271,7 @@
 static int prio_dump(struct Qdisc *sch, struct sk_buff *skb)
 {
 	struct prio_sched_data *q = qdisc_priv(sch);
-	unsigned char	 *b = skb->tail;
+	unsigned char *b = skb_tail_pointer(skb);
 	struct tc_prio_qopt opt;
 
 	opt.bands = q->bands;