net: sched: implement action-specific terse dump

Allow user to request action terse dump with new flag value
TCA_FLAG_TERSE_DUMP. Only output essential action info in terse dump (kind,
stats, index and cookie, if set by the user when creating the action). This
is different from filter terse dump where index is excluded (filter can be
identified by its own handle).

Move tcf_action_dump_terse() function to the beginning of source file in
order to call it from tcf_dump_walker().

Signed-off-by: Vlad Buslov <vlad@buslov.dev>
Suggested-by: Jamal Hadi Salim <jhs@mojatatu.com>
Acked-by: Cong Wang <xiyou.wangcong@gmail.com>
Link: https://lore.kernel.org/r/20201102201243.287486-1-vlad@buslov.dev
Signed-off-by: Jakub Kicinski <kuba@kernel.org>