ZTS: Fix reservation_001_pos

It's possible for an unrelated process, like blkid, to have the
volume open when 'zfs destroy' is run. Switch the cleanup function
to the destroy_dataset() helper which handles this case by retrying
the destroy when the dataset is busy.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #7796