OpenZFS 9914 - NV_UNIQUE_NAME_TYPE broken after 9580

OpenZFS 9914 - NV_UNIQUE_NAME_TYPE broken after 9580

Authored by: Andrew Stormont <astormont@racktopsystems.com>
Reviewed by: Yuri Pankov <yuripv@yuripv.net>
Reviewed by: Garrett D'Amore <garrett@damore.org>
Reviewed by: Andy Fiddaman <omnios@citrus-it.co.uk>
Reviewed-by: Serapheim Dimitropoulos <serapheim@delphix.com>
Reviewed-by: Igor Kozhukhov <igor@dilos.org>
Approved by: Dan McDonald <danmcd@joyent.com>
Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>
OpenZFS-issue: https://www.illumos.org/issues/9914
OpenZFS-commit: https://github.com/illumos/illumos-gate/commit/b8a5bee18
Closes #8496