net/flow_dissector: correct comments on enum flow_dissector_key_id

There are no such structs flow_dissector_key_flow_vlan or
 flow_dissector_key_flow_tags, the actual structs used are struct
 flow_dissector_key_vlan and struct flow_dissector_key_tags.  So correct the
 comments against FLOW_DISSECTOR_KEY_VLAN, FLOW_DISSECTOR_KEY_FLOW_LABEL and
 FLOW_DISSECTOR_KEY_CVLAN to refer to those.

Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>