MFV r325607: 8607 zfs: variable set but not used

illumos/illumos-gate@b852c2f54326f8ac1daa372a88bfe951dd7e20ed
https://github.com/illumos/illumos-gate/commit/b852c2f54326f8ac1daa372a88bfe951dd7e20ed

https://www.illumos.org/issues/8607

Reviewed by: Yuri Pankov <yuripv@gmx.com>
Reviewed by: Igor Kozhukhov <igor@dilos.org>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Author: Toomas Soome <tsoome@me.com>

MFC after: 1 week