netfilter: nft_payload: add C-VLAN offload support

Match on h_vlan_encapsulated_proto and set up protocol dependency. Check
for protocol dependency before accessing the tci field. Allow to match
on the encapsulated ethertype too.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>