flow_offload: add ops to tc_action_ops for flow action setup

Add a new ops to tc_action_ops for flow action setup.

Refactor function tc_setup_flow_action to use this new ops.

We make this change to facilitate to add standalone action module.

We will also use this ops to offload action independent of filter
in following patch.

Signed-off-by: Baowen Zheng <baowen.zheng@corigine.com>
Signed-off-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>