net: flow_dissector: Wrap unionized VLAN fields in a struct

In commit a82055af ("netfilter: nft_payload: add VLAN offload
support"), VLAN fields in struct flow_dissector_key_vlan were unionized
with the intention of introducing another field that covered the whole TCI
header. However without a wrapping struct the subfields end up sharing the
same bits. As a result, "tc filter add ... flower vlan_id 14" specifies not
only vlan_id, but also vlan_priority.

Fix by wrapping the individual VLAN fields in a struct.

Fixes: a82055af

 ("netfilter: nft_payload: add VLAN offload support")
Signed-off-by: Petr Machata <petrm@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>