netfilter: ebt_nflog: fix unexpected truncated packet

"struct nf_loginfo li;" is a local variable, so we should set the flags
to 0 explicitly, else, packets maybe truncated unexpectedly when copied
to the userspace.

Fixes: 7643507f ("netfilter: xt_NFLOG: nflog-range does not truncate packets")
Cc: Vishwanath Pai <vpai@akamai.com>
Signed-off-by: Liping Zhang <zlpnobody@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>