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: NPablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: NPatrick McHardy <kaber@trash.net>

net/netfilter/nf_conntrack_core.c

@@ -734,6 +734,8 @@ nf_conntrack_in(struct net *net, u_int8_t pf, unsigned int hooknum,
 		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;
 	}