net: flow: rename headers parse graph to smiply headers_graph

In reality what we describe is the relation between headers
so that user space can unambiguously "know" what header fields
are valid to match on and how deep the stack can possibly
go. So for example we can show QinQ is valid but QinQinQ is
not.

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

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