ZFS fails with error 6 on upgrade to 11.1

[Sebastian Schwarz]

On 2017-07-28, Goran Tepšić wrote:
> Hmm. Can you do 'zfs get bootfs,cachefile ssd'?

Sure:

> # zpool get bootfs,cachefile ssd
> NAME  PROPERTY   VALUE                    SOURCE
> ssd   bootfs     ssd/ROOT/freebsd11.0.11  local
> ssd   cachefile  -                        default

That bootfs is my currently working dataset.  I selected the
dataset with 11.1 using the boot loader's menu.

I tried setting bootfs to `ssd/ROOT/freebsd11.1.0` to see if
that would make a difference, but it didn't.