nfp: flower: link pre_tun flow rules with neigh entries

Add helper functions that can create links between flow rules
and cached neighbour entries. Also add the relevant calls to
these functions.

* When a new neighbour entry gets added cycle through the saved
  pre_tun flow list and link any relevant matches. Update the
  neighbour table on the nfp with this new information.
* When a new pre_tun flow rule gets added iterate through the
  save neighbour entries and link any relevant matches. Once
  again update the nfp neighbour table with any new links.
* Do the inverse when deleting - remove any created links and
  also inform the nfp of this.

Signed-off-by: Louis Peens <louis.peens@corigine.com>
Signed-off-by: Yinjun Zhang <yinjun.zhang@corigine.com>
Signed-off-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>