ZFS fails with error 6 on upgrade to 11.1

[Sebastian Schwarz]

I performed a fresh install of 11.0-RELEASE into a new boot
environment as well as another one of 11.1-RELEASE.  The 11.0
install boots, the 11.1 one doesn't.  Both were created using
the same procedure.  Therefore I presume it to be a bug in 11.1
and have filed a PR accordingly:
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=221075

Still any ideas to get 11.1 to boot off my ZFS pool are very
much appreciated.