sfc: neighbour lookup for TC encap action offload

For each neighbour we're interested in, create a struct efx_neigh_binder
 object which has a list of all the encap_actions using it.  When we
 receive a neighbouring update (through the netevent notifier), find the
 corresponding efx_neigh_binder and update all its users.
Since the actual generation of encap headers is still only a stub, the
 resulting rules still get left on fallback actions.

Signed-off-by: Edward Cree <ecree.xilinx@gmail.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Reviewed-by: Pieter Jansen van Vuuren <pieter.jansen-van-vuuren@amd.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>