netfilter: nft_ct: report error if mark and dir specified simultaneously

NFT_CT_MARK is unrelated to direction, so if NFTA_CT_DIRECTION attr is
specified, report EINVAL to the userspace. This validation check was
already done at nft_ct_get_init, but we missed it in nft_ct_set_init.

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