netfilter: nft_payload: add VLAN offload support

Match on ethertype 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>