ZTS: Cleanup partition tables

The cleanup_devices function should remove any partitions created
on the device and force the partition table to be reread. This
is needed to ensure that blkid has an up to date version of what
devices and partitions are used by zfs.

The cleanup_devices call was removed from inuse_008_pos.ksh since
it operated on partitions instead of devices and was not needed.

Lastly ddidecode may be called by parted and was therefore added
to the constrained path.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #9806