netfilter: nft_tunnel: Add NFTA_TUNNEL_MODE options

nft "tunnel" expr match both the tun_info of RX and TX. This patch
provide the NFTA_TUNNEL_MODE to individually match the tun_info of
RX or TX.

Signed-off-by: wenxu <wenxu@ucloud.cn>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>