Add (unfortunately overlong) TODO item for CVE-2018-18653

This seem at first glance a very specific issue for the Linux kernel as
shiped in Ubuntu. But there are some remarks pointed out via
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1798863 .

The issue seem introduced via an out-of-tree patch from the Lockdown
patchset:

https://git.launchpad.net/~ubuntu-kernel/ubuntu/+source/linux/+git/cosmic/commit/?id=03c7de9e956395f3b36f86f89b62780ad9501eef

https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1798863/comments/21
is further relevant to determine our status on which combinations of
used patch and used configuration options will cause the issue.