When using nftables with CONFIG_NETFILTER_XT_TARGET_TRACE=n, we get
lots of "TRACE: filter:output:policy:1 IN=..." warnings as several
places will leave skb->nf_trace uninitialised.

Unlike iptables tracing functionality is not conditional in nftables,
so always copy/zero nf_trace setting when nftables is enabled.

Move this into __nf_copy() helper.
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>