netfilter: reset nf_trace in nf_reset

We forgot to clear the nf_trace of sk_buff in nf_reset,
When we use veth device, this nf_trace information will
be leaked from one net namespace to another net namespace.

Signed-off-by: Gao feng <gaofeng@cn.fujitsu.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>