net: Make qdisc_skb_cb upper size bound explicit.

Just like skb->cb[], so that qdisc_skb_cb can be encapsulated inside
of other data structures.

This is intended to be used by IPoIB so that it can remember
addressing information stored at hard_header_ops->create() time that
it can fetch when the packet gets to the transmit routine.

Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/sched/sch_sfb.c b/net/sched/sch_sfb.c
index 96e42ca..d7eea99 100644
--- a/net/sched/sch_sfb.c
+++ b/net/sched/sch_sfb.c
@@ -94,8 +94,7 @@
 
 static inline struct sfb_skb_cb *sfb_skb_cb(const struct sk_buff *skb)
 {
-	BUILD_BUG_ON(sizeof(skb->cb) <
-		sizeof(struct qdisc_skb_cb) + sizeof(struct sfb_skb_cb));
+	qdisc_cb_private_validate(skb, sizeof(struct sfb_skb_cb));
 	return (struct sfb_skb_cb *)qdisc_skb_cb(skb)->data;
 }