flow_dissector: Add enums for encapsulation keys

New encapsulation keys were added to the flower classifier, which allow
classification according to outer (encapsulation) headers attributes
such as key and IP addresses.
In order to expose those attributes outside flower, add
corresponding enums in the flow dissector.

Signed-off-by: Hadar Hen Zion <hadarh@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>