[Cake] Backporting 'tc class' support

[Toke Høiland-Jørgensen]

Now that CAKE has been accepted upstream, I figured it was a good time
to backport the 'tc class' support. So I did, back to kernel v4.9.

This is in the master branch; anyone feel like testing? With this, the
version of CAKE in the master branch should be identical to the version
that will be in Linux 4.19 :)

-Toke