[Cake] Correct 'change' behaviour

[Toke Høiland-Jørgensen]

So we've seen a couple of issues with reconfiguration being weird (i.e.
retaining previous values). Looking at the code in cake_change() and the
iproute2 code, this seems to stem from the fact that tc only sets the
parameters (in the netlink message) that are actually specified on the
command line, and the kernel only changes the parameters specified in
the netlink message.

This leads to the different behaviour: On init, not specifying a
parameter means "give me a sensible default", whereas on a reconfigure
it means "don't change this parameter from its current value". This is
obviously inconsistent. I'd argue that the right thing to do is the
former in all cases; is there any reason why that would not be the case?
If not, I'll go fix it :)

-Toke