netfilter: ipset: Support the -exist flag with the destroy command

The -exist flag was supported with the create, add and delete commands.
In order to gracefully handle the destroy command with nonexistent sets,
the -exist flag is added to destroy too.
Signed-off-by: Jozsef Kadlecsik <kadlec@netfilter.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/ipset/ip_set_core.c

@@ -1239,9 +1239,11 @@ static int ip_set_destroy(struct net *net, struct sock *ctnl,
 		/* Modified by ip_set_destroy() only, which is serialized */
 		inst->is_destroyed = false;
 	} else {
+		u32 flags = flag_exist(nlh);
 		s = find_set_and_id(inst, nla_data(attr[IPSET_ATTR_SETNAME]),
 				    &i);
 		if (!s) {
+			if (!(flags & IPSET_FLAG_EXIST))
 				ret = -ENOENT;
 			goto out;
 		} else if (s->ref || s->ref_netlink) {