netfilter: ctnetlink: fix reliable event delivery if message building fails

This patch fixes a bug that allows to lose events when reliable
event delivery mode is used, ie. if NETLINK_BROADCAST_SEND_ERROR
and NETLINK_RECV_NO_ENOBUFS socket options are set.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/netfilter/nf_conntrack_netlink.c b/net/netfilter/nf_conntrack_netlink.c
index 2b2af63..569410a 100644
--- a/net/netfilter/nf_conntrack_netlink.c
+++ b/net/netfilter/nf_conntrack_netlink.c
@@ -582,7 +582,9 @@
 nlmsg_failure:
 	kfree_skb(skb);
 errout:
-	nfnetlink_set_err(net, 0, group, -ENOBUFS);
+	if (nfnetlink_set_err(net, 0, group, -ENOBUFS) > 0)
+		return -ENOBUFS;
+
 	return 0;
 }
 #endif /* CONFIG_NF_CONNTRACK_EVENTS */