Illumos 6537 - Panic on zpool scrub with DEBUG kernel

6537 Panic on zpool scrub with DEBUG kernel
Reviewed by: Steve Gonczi <gonczi@comcast.net>
Reviewed by: Dan McDonald <danmcd@omniti.com>
Reviewed by: Igor Kozhukhov <ikozhukhov@gmail.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Approved by: Matthew Ahrens <mahrens@delphix.com>

References:

https://www.illumos.org/issues/6537
https://github.com/illumos/illumos-gate/commit/8c04a1f

Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>