[][Add hnat count to nf_conntrack]

[Description]
Add hnat count to nf_conntrack through keepalive.
Notice that we need to enable it by command:
echo "7 1" > /sys/kernel/debug/hnat/hnat_setting

Test Result:
root@OpenWrt:/# conntrack -L
tcp 6 7439 ESTABLISHED ... packets=126876 bytes=192176221 ... packets=27121 bytes=1629422

After keepalive interval, "conntrack -L" again, we could see hnat counter is updated to conntrack.
root@OpenWrt:/# conntrack -L
tcp 6 7439 ESTABLISHED ... packets=596148 bytes=902807461 ... packets=116947 bytes=7025906

[Release-log]
N/A

Change-Id: I189a8f8a5a332b67f00b8fc922a1d5c5343acde8
Reviewed-on: https://gerrit.mediatek.inc/c/openwrt/feeds/mtk_openwrt_feeds/+/5211150
3 files changed