MFV 286604: 5812 assertion failed in zrl_tryenter(): zr_owner==NULL

Reviewed by: George Wilson <george@delphix.com>
Reviewed by: Alex Reece <alex@delphix.com>
Reviewed by: Will Andrews <will@freebsd.org>
Approved by: Gordon Ross <gwr@nexenta.com>
Author: Matthew Ahrens <mahrens@delphix.com>

illumos/illumos-gate@8df173054ca442cd8845a7364c3edad9d6822351