netfilter: nf_conntrack: fix event flooding in GRE protocol tracker

GRE connections cause ctnetlink event flood because the ASSURED event
is set for every packet received.
Reported-by: Denys Fedoryshchenko <denys@visp.net.lb>
Tested-by: Denys Fedoryshchenko <denys@visp.net.lb>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_conntrack_proto_gre.c

@@ -241,8 +241,8 @@ static int gre_packet(struct nf_conn *ct,
 		nf_ct_refresh_acct(ct, ctinfo, skb,
 				   ct->proto.gre.stream_timeout);
 		/* Also, more likely to be important, and not a probe. */
-		set_bit(IPS_ASSURED_BIT, &ct->status);
-		nf_conntrack_event_cache(IPCT_ASSURED, ct);
+		if (!test_and_set_bit(IPS_ASSURED_BIT, &ct->status))
+			nf_conntrack_event_cache(IPCT_ASSURED, ct);
 	} else
 		nf_ct_refresh_acct(ct, ctinfo, skb,
 				   ct->proto.gre.timeout);