sfc: some plumbing towards TC encap action offload

Create software objects to manage the metadata for encap actions that
 can be attached to TC rules.  However, since we don't yet have the
 neighbouring information (needed to generate the Ethernet header),
 all rules with encap actions are marked as "unready" and thus insert
 the fallback action into hardware rather than actually offloading the
 encapsulation action.

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