netfilter: nfnetlink_log: fix per-rule qthreshold override

In NFLOG the per-rule qthreshold should overrides per-instance only
it is set. With current code, the per-rule qthreshold is 1 if not set
and it overrides the per-instance qthreshold.

This patch modifies the default xt_NFLOG threshold from 1 to
0. Thus a value of 0 means there is no per-rule setting and the instance
parameter has to apply.

Signed-off-by: Eric Leblond <eric@inl.fr>
Signed-off-by: Patrick McHardy <kaber@trash.net>