netfilter: nftables_offload: special ethertype handling for VLAN

netfilter: nftables_offload: special ethertype handling for VLAN

The nftables offload parser sets FLOW_DISSECTOR_KEY_BASIC .n_proto to the
ethertype field in the ethertype frame. However:

- FLOW_DISSECTOR_KEY_BASIC .n_proto field always stores either IPv4 or IPv6
  ethertypes.
- FLOW_DISSECTOR_KEY_VLAN .vlan_tpid stores either the 802.1q and 802.1ad
  ethertypes. Same as for FLOW_DISSECTOR_KEY_CVLAN.

This function adjusts the flow dissector to handle two scenarios:

1) FLOW_DISSECTOR_KEY_VLAN .vlan_tpid is set to 802.1q or 802.1ad.
   Then, transfer:
   - the .n_proto field to FLOW_DISSECTOR_KEY_VLAN .tpid.
   - the original FLOW_DISSECTOR_KEY_VLAN .tpid to the
     FLOW_DISSECTOR_KEY_CVLAN .tpid
   - the original FLOW_DISSECTOR_KEY_CVLAN .tpid to the .n_proto field.

2) .n_proto is set to 802.1q or 802.1ad. Then, transfer:
   - the .n_proto field to FLOW_DISSECTOR_KEY_VLAN .tpid.
   - the original FLOW_DISSECTOR_KEY_VLAN .tpid to the .n_proto field.

Fixes: 2850100d7e2c ("netfilter: nft_payload: add VLAN offload support")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>