netfilter: ipset: fix ip_set_flush return code

ip_set_flush returned -EPROTO instead of -IPSET_ERR_PROTOCOL, fixed
Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/ipset/ip_set_core.c

@@ -815,7 +815,7 @@ ip_set_flush(struct sock *ctnl, struct sk_buff *skb,
 	ip_set_id_t i;
 
 	if (unlikely(protocol_failed(attr)))
-		return -EPROTO;
+		return -IPSET_ERR_PROTOCOL;
 
 	if (!attr[IPSET_ATTR_SETNAME]) {
 		for (i = 0; i < ip_set_max; i++)