Illumos 934 - FreeBSD's GPT not recognized

Reviewed by: Alexander Eremin <alexander.r.eremin@gmail.com>
Reviewed by: Garrett D'Amore <garrett@damore.org>
Reviewed by: Andrew Stormont <Andrew.Stormont@nexenta.com>
Reviewed by: Richard Elling <richard.elling@richardelling.com>
Approved by: Gordon Ross <gwr@nexenta.com>

References:

https://www.illumos.org/issues/934
https://github.com/illumos/illumos-gate/commit/e21ea67

Ported-by: ilovezfs <ilovezfs@icloud.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #4016