netfilter: nf_conntrack: account packets drop by tcp_packet()

Since tcp_packet() may return -NF_DROP in two situations, the
packet-drop stats must be increased.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Patrick McHardy <kaber@trash.net>
diff --git a/net/netfilter/nf_conntrack_core.c b/net/netfilter/nf_conntrack_core.c
index 2235432..ebc2756 100644
--- a/net/netfilter/nf_conntrack_core.c
+++ b/net/netfilter/nf_conntrack_core.c
@@ -734,6 +734,8 @@
 		nf_conntrack_put(skb->nfct);
 		skb->nfct = NULL;
 		NF_CT_STAT_INC_ATOMIC(net, invalid);
+		if (ret == -NF_DROP)
+			NF_CT_STAT_INC_ATOMIC(net, drop);
 		return -ret;
 	}