OpenZFS 9479 - fix wrong format specifier for vdev_id

Authored by: Andriy Gapon <avg@FreeBSD.org>
Reviewed by: Matt Ahrens <matt@delphix.com>
Reviewed by: Giuseppe Di Natale <guss80@gmail.com>
Reviewed by: George Melikov <mail@gmelikov.ru>
Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>
Approved by: Robert Mustacchi <rm@joyent.com>

OpenZFS-issue: https://www.illumos.org/issues/9479
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/20aa447c
Closes #7699