Illumos 5812 - assertion failed in zrl_tryenter(): zr_owner==NULL

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>

References:

https://www.illumos.org/issues/5812
https://github.com/illumos/illumos-gate/commit/8df1730

Ported-by: DHE <git@dehacked.net>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #3357