ZFS performance tests should clean up NFS mount
This change umounts client's NFS mount after each test so we can avoid
two sporadic issues:
- client NFS stale mount and
- zpool export and zpool destroy failed due to dataset busy
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tony Nguyen <tony.nguyen@delphix.com>
Closes #10767