net: flow: various message tweaks.

Add an instance indicator to support multiple instances
of the same header in the headers graph.

Remove containers for sets of tables, headers, etc. They are not
needed and I find it simplier to simply null terminate lists.

Add some #defines to ixgbe_pipeline but will fix code generator
to do a better job instead of further cleanup the pipeline by
hand.

Signed-off-by: John Fastabend <john.r.fastabend@intel.com>
net: flow: fix put_flow semantics

Put_flow is broke and will in fact cuase a null pointer
dereference in many cases. At some point it must have
worked but no longer.

This patch fixes it up.

Left to do, stop using magic numbers for header/field
definitions. Clean up code some more.

Signed-off-by: John Fastabend <john.r.fastabend@intel.com>