Add SKB Priority qdisc support in tc(8)

Add SKB Priority qdisc support in tc(8)

sch_skbprio is a qdisc that prioritizes packets according to their skb->priority
field. Under congestion, it drops already-enqueued lower priority packets to
make space available for higher priority packets. Skbprio was conceived as a
solution for denial-of-service defenses that need to route packets with
different priorities as a means to overcome DoS attacks.

Signed-off-by: Nishanth Devarajan <ndev2021@gmail.com>
Reviewed-by: Michel Machado <michel@digirati.com.br>
Signed-off-by: David Ahern <dsahern@gmail.com>