unix: Use FIELD_SIZEOF() in af_unix_init().

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/unix/af_unix.c

@@ -2426,9 +2426,8 @@ static struct pernet_operations unix_net_ops = {
 static int __init af_unix_init(void)
 {
 	int rc = -1;
-	struct sk_buff *dummy_skb;
 
-	BUILD_BUG_ON(sizeof(struct unix_skb_parms) > sizeof(dummy_skb->cb));
+	BUILD_BUG_ON(sizeof(struct unix_skb_parms) > FIELD_SIZEOF(struct sk_buff, cb));
 
 	rc = proto_register(&unix_proto, 1);
 	if (rc != 0) {