netfilter: conntrack: fix dropping packet after l4proto->packet()

We currently use the negative value in the conntrack code to encode
the packet verdict in the error. As NF_DROP is equal to 0, inverting
NF_DROP makes no sense and, as a result, no packets are ever dropped.

Signed-off-by: Christoph Paasch <christoph.paasch@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Patrick McHardy <kaber@trash.net>
diff --git a/net/netfilter/nf_conntrack_proto_tcp.c b/net/netfilter/nf_conntrack_proto_tcp.c
index a1edb9c..f3fd154 100644
--- a/net/netfilter/nf_conntrack_proto_tcp.c
+++ b/net/netfilter/nf_conntrack_proto_tcp.c
@@ -859,7 +859,7 @@
 			 */
 			if (nf_ct_kill(ct))
 				return -NF_REPEAT;
-			return -NF_DROP;
+			return NF_DROP;
 		}
 		/* Fall through */
 	case TCP_CONNTRACK_IGNORE:
@@ -892,7 +892,7 @@
 				nf_log_packet(pf, 0, skb, NULL, NULL, NULL,
 					  "nf_ct_tcp: killing out of sync session ");
 			nf_ct_kill(ct);
-			return -NF_DROP;
+			return NF_DROP;
 		}
 		ct->proto.tcp.last_index = index;
 		ct->proto.tcp.last_dir = dir;