Illumos #3189 kernel panic in test hotspare_onoffline_004_neg

3189 kernel panic in ZFS test suite during hotspare_onoffline_004_neg

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Arne Jansen <sensille@gmx.net>
Approved by: Dan McDonald <danmcd@nexenta.com>

References:

illumos/illumos-gate@8f0b538d1dc99df23a6a89cfd9ffddc1b9804a00
changeset: 13818:e9ad0a945d45
https://www.illumos.org/issues/3189

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