netfilter: nft_tunnel: add support for geneve opts

netfilter: nft_tunnel: add support for geneve opts

Like vxlan and erspan opts, geneve opts should also be supported in
nft_tunnel. The difference is geneve RFC (draft-ietf-nvo3-geneve-14)
allows a geneve packet to carry multiple geneve opts. So with this
patch, nftables/libnftnl would do:

  # nft add table ip filter
  # nft add chain ip filter input { type filter hook input priority 0 \; }
  # nft add tunnel filter geneve_02 { type geneve\; id 2\; \
    ip saddr 192.168.1.1\; ip daddr 192.168.1.2\; \
    sport 9000\; dport 9001\; dscp 1234\; ttl 64\; flags 1\; \
    opts \"1:1:34567890,2:2:12121212,3:3:1212121234567890\"\; }
  # nft list tunnels table filter
    table ip filter {
    	tunnel geneve_02 {
    		id 2
    		ip saddr 192.168.1.1
    		ip daddr 192.168.1.2
    		sport 9000
    		dport 9001
    		tos 18
    		ttl 64
    		flags 1
    		geneve opts 1:1:34567890,2:2:12121212,3:3:1212121234567890
    	}
    }

v1->v2:
  - no changes, just post it separately.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>