[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/packet/af_packet.c

@@ -775,7 +775,7 @@ static int packet_sendmsg(struct kiocb *iocb, struct socket *sock,
 		err = -EINVAL;
 		res = dev->hard_header(skb, dev, ntohs(proto), addr, NULL, len);
 		if (sock->type != SOCK_DGRAM) {
-			skb->tail = skb->data;
+			skb_reset_tail_pointer(skb);
 			skb->len = 0;
 		} else if (res < 0)
 			goto out_free;