ZTS: user_property_002_pos fails to destroy volume

During the cleanup function of this test, an attempt to destroy a volume
can fail because the volume is busy. This leaves the system with
unexpected datasets which in turn causes subsequent failures.

Reviewed-by: bunder2015 <omfgbunder@gmail.com>
Reviewed-by: Igor Kozhukhov <igor@dilos.org>
Reviewed-by: Giuseppe Di Natale <guss80@gmail.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: John Kennedy <john.kennedy@delphix.com>
Closes #8422