MFC r294805: MFV r294804:
6386 Fix function call with uninitialized value in vdev_inuse

Reviewed by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Dan Kimmel <dan.kimmel@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Author: Richard Yao <ryao@gentoo.org>

illumos/illumos-gate@5bdd995ddb777f538bfbcc5e2d5ff1bed07ae56e