flow dissector: ICMP support

Allow dissection of ICMP(V6) type and code. This should only occur
if a packet is ICMP(V6) and the dissector has FLOW_DISSECTOR_KEY_ICMP set.

There are currently no users of FLOW_DISSECTOR_KEY_ICMP.
A follow-up patch will allow FLOW_DISSECTOR_KEY_ICMP to be used by
the flower classifier.

Signed-off-by: Simon Horman <simon.horman@netronome.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>