I'm seeing an issue with Cake and a simple eBPF filter that doesn't happen
for me with fq_codel or sfq.

I tested with Ubuntu 18.04.1 LTS on VMWare Fusion:
root@ubuntu:~# uname -a
Linux ubuntu 4.15.0-30-generic #32-Ubuntu SMP Thu Jul 26 17:42:43 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux

How to reproduce (assuming Cake from head is already installed):

0) Run make to build simple-bpf.o.
1) Edit qos.sh to set IFACE.
2) At this point, make sure your system is prepared for a crash.
3) Run qos.sh (as root).
4) Send traffic over IFACE (ping should do it, or iperf3).
5) Within a few seconds, a crash occurs.

Notes:

- In VMWare I usually see "CPU disabled by guest" and the vmware-vmx
  process goes to either 100% (one core) or if I'm lucky 400% (four cores).
  I'm not sure if this is a panic, infinite loop or what.
- Oddly, I don't see it happen if I first run qos.sh with fq_codel or sfq
  (you can set QDISC in qos.sh), *then* run qos.sh with cake. It happens
  when I run it with cake right after a reboot.
- Happens with or without a cake bandwidth limit.
- Happens when minor classid is set to, at least, either 1 or 2 in simple-bpf.c
- Watch out, in case there's something simple wrong with my BPF or the way
  I add it with tc! Even still, I wouldn't expect to be able to cause this...
