netfilter: xt_NFLOG: fix unexpected truncated packet

Justin and Chris spotted that iptables NFLOG target was broken when they
upgraded the kernel to 4.8: "ulogd-2.0.5- IPs are no longer logged" or
"results in segfaults in ulogd-2.0.5".

Because "struct nf_loginfo li;" is a local variable, and flags will be
filled with garbage value, not inited to zero. So if it contains 0x1,
packets will not be logged to the userspace anymore.

Fixes: 7643507f ("netfilter: xt_NFLOG: nflog-range does not truncate packets")
Reported-by: Justin Piszcz <jpiszcz@lucidpixels.com>
Reported-by: Chris Caputo <ccaputo@alt.net>
Tested-by: Chris Caputo <ccaputo@alt.net>
Signed-off-by: Liping Zhang <liping.zhang@spreadtrum.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>