net: sched: act_csum: get rid of tcf_csum_walker and tcf_csum_search

tcf_csum_walker() and tcf_csum_search() do the same thing as generic
walk/search function, so remove them.

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>