[NET]: Decrease skb->cb[] to 40 bytes.

No control block usage, even on 64-bit, needs
the full current 48 bytes.  This brings sk_buff
size down to 256 bytes on 64-bit platforms and
fixes some performance regressions due to the
addition of the input_dev member.
Signed-off-by: David S. Miller <davem@redhat.com>

include/linux/skbuff.h

@@ -233,7 +233,7 @@ struct sk_buff {
 	 * want to keep them across layers you have to do a skb_clone()
 	 * first. This is owned by whoever has the skb queued ATM.
 	 */
-	char			cb[48];
+	char			cb[40];
 
 	unsigned int		len,
 				data_len,