openvswitch: Use FIELD_SIZEOF() in dp_init().

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

net/openvswitch/datapath.c

@@ -1989,10 +1989,9 @@ static struct pernet_operations ovs_net_ops = {
 
 static int __init dp_init(void)
 {
-	struct sk_buff *dummy_skb;
 	int err;
 
-	BUILD_BUG_ON(sizeof(struct ovs_skb_cb) > sizeof(dummy_skb->cb));
+	BUILD_BUG_ON(sizeof(struct ovs_skb_cb) > FIELD_SIZEOF(struct sk_buff, cb));
 
 	pr_info("Open vSwitch switching datapath\n");