netfilter: nf_tables_offload: allow ethernet interface type only

Hardware offload support at this stage assumes an ethernet device in
place. The flow dissector provides the intermediate representation to
express this selector, so extend it to allow to store the interface
type. Flower does not uses this, so skb_flow_dissect_meta() is not
extended to match on this new field.

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