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

tests/zfs-tests/tests/functional/reservation/reservation_001_pos.sh

@@ -54,7 +54,7 @@ verify_runnable "both"
 function cleanup
 {
 	for obj in $OBJ_LIST; do
-		datasetexists $obj && log_must zfs destroy -f $obj
+		destroy_dataset $obj
 	done
 }