netfilter: nft_log: fix snaplen does not truncate packets

There's a similar problem in xt_NFLOG, and was fixed by commit 7643507f
("netfilter: xt_NFLOG: nflog-range does not truncate packets"). Only set
copy_len here does not work, so we should enable NF_LOG_F_COPY_LEN also.

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