net/sched: cls_api: complement tcf_tfilter_dump_policy

In function `tc_dump_tfilter`, the attributes array is parsed via
tcf_tfilter_dump_policy which only describes TCA_DUMP_FLAGS. However,
the NLA TCA_CHAIN is also accessed with `nla_get_u32`.

The access to TCA_CHAIN is introduced in commit 5bc17018 ("net:
sched: introduce multichain support for filters") and no nla_policy is
provided for parsing at that point. Later on, tcf_tfilter_dump_policy is
introduced in commit f8ab1807 ("net: sched: introduce terse dump
flag") while still ignoring the fact that TCA_CHAIN needs a check. This
patch does that by complementing the policy to allow the access
discussed here can be safe as other cases just choose rtm_tca_policy as
the parsing policy.

Signed-off-by: Lin Ma <linma@zju.edu.cn>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>