net: sched: act_api: implement generic walker and search for tc action

Being able to get tc_action_net by using net_id stored in tc_action_ops
and execute the generic walk/search function, add __tcf_generic_walker()
and __tcf_idr_search() helpers.

Signed-off-by: Zhengchao Shao <shaozhengchao@huawei.com>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>