netfilter: use NF_DROP instead of -NF_DROP

At the beginning in 2009 one patch [1] introduced collecting drop
counter in nf_conntrack_in() by returning -NF_DROP. Later, another
patch [2] changed the return value of tcp_packet() which now is
renamed to nf_conntrack_tcp_packet() from -NF_DROP to NF_DROP. As
we can see, that -NF_DROP should be corrected.

Similarly, there are other two points where the -NF_DROP is used.

Well, as NF_DROP is equal to 0, inverting NF_DROP makes no sense
as patch [2] said many years ago.

[1]
commit 7d1e0459 ("netfilter: nf_conntrack: account packets drop by tcp_packet()")
[2]
commit ec8d5409 ("netfilter: conntrack: fix dropping packet after l4proto->packet()")
Signed-off-by: Jason Xing <kernelxing@tencent.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/ipv4/netfilter/iptable_filter.c

@@ -44,7 +44,7 @@ static int iptable_filter_table_init(struct net *net)
 		return -ENOMEM;
 	/* Entry 1 is the FORWARD hook */
 	((struct ipt_standard *)repl->entries)[1].target.verdict =
-		forward ? -NF_ACCEPT - 1 : -NF_DROP - 1;
+		forward ? -NF_ACCEPT - 1 : NF_DROP - 1;
 
 	err = ipt_register_table(net, &packet_filter, repl, filter_ops);
 	kfree(repl);

net/ipv6/netfilter/ip6table_filter.c

@@ -43,7 +43,7 @@ static int ip6table_filter_table_init(struct net *net)
 		return -ENOMEM;
 	/* Entry 1 is the FORWARD hook */
 	((struct ip6t_standard *)repl->entries)[1].target.verdict =
-		forward ? -NF_ACCEPT - 1 : -NF_DROP - 1;
+		forward ? -NF_ACCEPT - 1 : NF_DROP - 1;
 
 	err = ip6t_register_table(net, &packet_filter, repl, filter_ops);
 	kfree(repl);

net/netfilter/nf_conntrack_core.c

@@ -2024,7 +2024,7 @@ nf_conntrack_in(struct sk_buff *skb, const struct nf_hook_state *state)
 			goto repeat;
 
 		NF_CT_STAT_INC_ATOMIC(state->net, invalid);
-		if (ret == -NF_DROP)
+		if (ret == NF_DROP)
 			NF_CT_STAT_INC_ATOMIC(state->net, drop);
 
 		ret = -ret;