overlayfs breaks AppArmor

Reference: https://bugs.launchpad.net/apparmor/+bug/1408106

At March, 2015 AppArmor meeting:

jjohansen: overlayfs is currently just broken, and we are going to be working with upstream to try to get it fixed
darix: are the current issues documented somewhere?
jjohansen: upstream has already begun working on fixing many of the issues involved we just need to make sure we are on top of it and providing them feedback, and maybe a patch or two if needed
jjohansen: darix: only sort of in the lkml/fsdevel threads around the issues
jjohansen: it affect more than just apparmor
[...]
jjohansen: to summarize, basically overlayfs took some short cuts and some places the hooks see the upper (overlayfs) dentry/vfsmnt
jjohansen: and some places only see the lower dentry/vfsmnt (which is also a private clone mnt)
tyhicks: darix: this is a decent placeholder bug to follow for the general overlayfs issue: https://bugs.launchpad.net/apparmor/+bug/1408106
jjohansen: once the overlayfs issues are fixed we should be good with doing unioning via overlayfs
intrigeri: be sure that I'll (have to) test it, including with multiple lower-layers
[...]
jjohansen: intrigeri: yeah we will have to test it too, there are several projects that want to use it

Blueprint: https://tails.boum.org/contribute/design/application_isolation/#overlayfs

Feature Branch: feature/8415-overlayfs-stretch

Parent Task: #8415

Original created by @intrigeri on 9045 (Redmine)