overlayroot is incompatible with util-linux's new "libmount-mountfd" code

[Michael Hudson-Doyle]

We discovered a regression in MAAS images very late in the mantic cycle: https://bugs.launchpad.net/bugs/2037417

This turned out to be caused by new code in util-linux, so we turned that off: https://launchpad.net/ubuntu/+source/util-linux/2.39.1-4ubuntu2

Since then there has been a bit of discussion in some upstream reports:

https://github.com/util-linux/util-linux/issues/2528
https://github.com/util-linux/util-linux/issues/2576

It's not clear what the right way forward is but it's possible that a change to overlayroot will help enable the new code in util-linux eventually.