netfilter: nft_meta: set skb->nf_trace appropriately

When user add a nft rule to set nftrace to zero, for example:

  # nft add rule ip filter input nftrace set 0

We should set nf_trace to zero also.

Signed-off-by: Liping Zhang <liping.zhang@spreadtrum.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>