Disc full during upgrade with / on btrfs fs and using docker (subvolumes)

[Jan Katins]

I experienced a disc full event on a `do-release-upgrade` when upgrading my ubuntu 17.10 to 18.04 with my root partition on a btrfs fs. `df` reported 50% free (= a lie), but apparently docker created so man subvolumes (which apparently df doesn't know about) that the disc was almost full.

The upgrade crashed while installing packages taking some of my running GUI apps with them and not allowing me to start any new ones. I had to boot into a rescue console and in the end deleted all docker subvolumes in a for loop:

```
$ btrfs subvolume list /
ID 338 gen 94967 top level 5 path var/lib/docker/btrfs/subvolumes/0d2bd1457a1a6ed78b1b3be1c1ff57cc1c709b1f4b25edc27c6d31ce1bbee8c8
ID 339 gen 94969 top level 5 path
[...]
$ # remove the echo...
$ for subvol in $(ls /var/lib/docker/btrfs/subvolumes); do echo btrfs subvolume delete $subvol ; done
$ btrfs balance start /
```

Afterwards running `apt install -f` and whatever it asked me to run (some dpkg stuff to finish the installs) got me back into a running system.

-> Please check that a btrfs root partition has enough left space to support a upgrade with e.g. `btrfs fi show /` and warn if a out-of-space situation is likely.