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>