6385 Fix unlocking order in zfs_zget

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

illumos/illumos-gate@eaef6a96de3f6afbbccc69bd7a0aed4463689d0a