flow_offload: add index to flow_action_entry structure

Add index to flow_action_entry structure and delete index from police and
gate child structure.

We make this change to offload tc action for driver to identify a tc
action.

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>