flow_dissector: allow dissection of tunnel options from metadata

Allow the existing 'dissection' of tunnel metadata to 'dissect'
options already present in tunnel metadata. This dissection is
controlled by a new dissector key, FLOW_DISSECTOR_KEY_ENC_OPTS.

This dissection only occurs when skb_flow_dissect_tunnel_info()
is called, currently only the Flower classifier makes that call.
So there should be no impact on other users of the flow dissector.

This is in preparation for allowing the flower classifier to
match on Geneve options.

Signed-off-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: Pieter Jansen van Vuuren <pieter.jansenvanvuuren@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>