Ubuntu 18.04 kernel 5.4 btrfs bug prevents btrbk from running

Just wanted to inform other users of btrbk that there is currently a bug in Ubuntu 18.04 with kernel 5.4 that prevents btrbk from running because of sporadic segfaults on "btrfs subvolume list", which is still unfixed. Btrbk then fails on affected drives with log message:

`!!! Target "/mnt/mountpoint" aborted: Failed to fetch subvolume detail`

Bug report: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1901755

Workaround: Install current mainline kernel (Description: https://wiki.ubuntu.com/Kernel/MainlineBuilds)

Start with the Ubuntu Launchpad report linked in the issue and reproduce the sporadic failure of `btrfs subvolume list` on Ubuntu 18.04 with kernel 5.4. Compare it with btrbk's `Failed to fetch subvolume detail` message; done would require a clearly defined btrbk change or documentation update, which this report does not currently specify.

Written by the indexing model from the issue text.