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.

v5:
*Update the drop counter for TC_ACT_SHOT

v4:
*Not allow setting flags other than the expected ones.

*Allow dumping the pure flags.

v3:
*Use optional flags, so that it won't break old versions of tc.

*Allow users to set both SKBEDIT_F_PRIORITY and SKBEDIT_F_INHERITDSFIELD flags.

v2:
*Fix the style issue

*Move the code from skbmod to skbedit

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>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Reviewed-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Acked-by: Davide Caratti <dcaratti@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>