l2tp: avoid precidence issues in L2TP_SKB_CB macro

checkpatch warned about the L2TP_SKB_CB macro's use of its argument: add
braces to avoid the problem.
Signed-off-by: Tom Parkin <tparkin@katalix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/l2tp/l2tp_core.c

@@ -93,7 +93,7 @@ struct l2tp_skb_cb {
 	unsigned long		expires;
 };
 
-#define L2TP_SKB_CB(skb)	((struct l2tp_skb_cb *)&skb->cb[sizeof(struct inet_skb_parm)])
+#define L2TP_SKB_CB(skb)	((struct l2tp_skb_cb *)&(skb)->cb[sizeof(struct inet_skb_parm)])
 
 static struct workqueue_struct *l2tp_wq;