net/sched: cls_api: add helper for tc cls walker stats dump

The walk implementation of most tc cls modules is basically the same.
That is, the values of count and skip are checked first. If count is
greater than or equal to skip, the registered fn function is executed.
Otherwise, increase the value of count. So we can reconstruct them.

Signed-off-by: Zhengchao Shao <shaozhengchao@huawei.com>
Reviewed-by: Jamal Hadi Salim <jhs@mojatatu.com>
Reviewed-by: Victor Nogueira <victor@mojatatu.com>
Tested-by: Victor Nogueira <victor@mojatatu.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>