netfilter: nft_payload: fix C-VLAN offload support

- add another struct flow_dissector_key_vlan for C-VLAN
- update layer 3 dependency to allow to match on IPv4/IPv6

Fixes: 89d8fd44

 ("netfilter: nft_payload: add C-VLAN offload support")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>