net:sched: add action inheritdsfield to skbedit

net:sched: add action inheritdsfield to skbedit

The new action inheritdsfield copies the field DS of
IPv4 and IPv6 packets into skb->priority. This enables
later classification of packets based on the DS field.

v4:
* Make tc use netlink helper functions

v3:
* Make flag represented in JSON output as a null value

v2:
* Align the output syntax with the input syntax

* Fix the style issues

Original idea by Jamal Hadi Salim <jhs@mojatatu.com>

Signed-off-by: Qiaobin Fu <qiaobinf@bu.edu>
Reviewed-by: Michel Machado <michel@digirati.com.br>
Reviewed-by: Cong Wang <xiyou.wangcong@gmail.com>
Reviewed-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Reviewed-by: Stephen Hemminger <stephen@networkplumber.org>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David Ahern <dsahern@gmail.com>