ZFS fails with error 6 on upgrade to 11.1

[Sebastian Schwarz]

On 2017-07-29, Goran Tepšić wrote:
> Let me know when you sort it out, I'd like to know what
> happened in case i need it sometimes god forbid :)

I'm beginning to think that my ZFS pool and datasets are just
fine.  Booting the 11.1 installer from a USB drive allows me to
access all the datasets from my pool without any problems.

I guess something is wrong with the device discovery during
early boot.  Which is confusing, since all the ZFS pool's
devices are present in the "List of GEOM managed disk devices"
(printed by pressing "?" at the "mountroot>" prompt).