kmod-sched-cake: update & restore DSCP washing

kmod-sched-cake: update & restore DSCP washing

Track upstream changes, incl changes in packet overhead accounting
(automatically taking care of linux' packet sizing knowledge),
improvements to triple isolated DRR handling (new flow dominance),
statistics tweak & allow more packet drops in stressed conditions.

Under tests this has significantly improved latency control under
'many flows to one' scenarious as is typical of bittorrent and MS
Windows update.

I also restored 'DSCP washing' functionality in my repo which follows
upstream closely (like a hawk!) with tc keywords 'wash/nowash'.  This
allows cake to limit/control packets in bands determined by a packet's
DSCP but to clear those DSCP bits on qdisc egress.  This functionality
was originally removed as part of an attempt to push cake into the
kernel, which hasn't actually happened as yet.

A matching commit is required to iproute2/tc to support the new overhead
handling, keyword changes as well as the 'wash/nowash' tweak.

Signed-off-by: Kevin Darbyshire-Bryant <kevin@darbyshire-bryant.me.uk>