netfilter: nft_tunnel: Add dst_cache support

The metadata_dst does not initialize the dst_cache field, this causes
problems to ip_md_tunnel_xmit() since it cannot use this cache, hence,
Triggering a route lookup for every packet.

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