[Cake] Parsing DSCP out of skb->priority

[Matt Corallo <cltaapptfwbs@mattcorallo.com>]

Its (presumably) not uncommon to have multiple encapsulation tunnels running on a host which then 
wants to do (cake) shaping on the actual outbound interface. In this case, the skb->hash is 
(hopefully) preserved from the original packet to select the right flow queue, but the DSCP isn't. 
This is ideally what the priority field of the skb is for - `skbedit` can set the priority flag of 
the packet before "exiting" the tunnel interface, and cake can ultimately read that priority flag to 
select a tin. However, doing so requires manual tin mapping in skbedit, reading the DSCP field and 
mapping them to the right tin entirely be hand.

skbedit also supports `inheritdsfield` which just sets skb->priority to the DSCP field as-is. It 
would be nice if cake could read such priority fields, treating skb->priority as a DSCP when 
TC_H_MAJ(skb->priority) is zero (which it always will be in this case). Is there any interest in a 
patch to do so? Would such a patch need a new userspace option to disable it, or would wash suffice?