[TCP]: Use BUILD_BUG_ON for tcp_skb_cb size checking

The sizeof(struct tcp_skb_cb) should not be less than the
sizeof(skb->cb). This is checked in net/ipv4/tcp.c, but
this check can be made more gracefully.
Signed-off-by: NPavel Emelyanov <xemul@openvz.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/tcp.c

@@ -2563,7 +2563,6 @@ void tcp_done(struct sock *sk)
 }
 EXPORT_SYMBOL_GPL(tcp_done);
 
-extern void __skb_cb_too_small_for_tcp(int, int);
 extern struct tcp_congestion_ops tcp_reno;
 
 static __initdata unsigned long thash_entries;
@@ -2582,9 +2581,7 @@ void __init tcp_init(void)
 	unsigned long limit;
 	int order, i, max_share;
 
-	if (sizeof(struct tcp_skb_cb) > sizeof(skb->cb))
-		__skb_cb_too_small_for_tcp(sizeof(struct tcp_skb_cb),
-					   sizeof(skb->cb));
+	BUILD_BUG_ON(sizeof(struct tcp_skb_cb) > sizeof(skb->cb));
 
 	tcp_hashinfo.bind_bucket_cachep =
 		kmem_cache_create("tcp_bind_bucket",