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>
Author: Gary Mills <gary_mills@fastmail.fm>

illumos/illumos-gate@8c04a1fa3f7d569d48fe9b5342d0bd4c533179b9