ZTS: Fix reservation_017_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: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #7750

tests/zfs-tests/tests/functional/reservation/reservation_017_pos.sh

@@ -55,7 +55,7 @@ function cleanup
 	typeset vol
 
 	for vol in $regvol $sparsevol; do
-		datasetexists $vol &&  log_must zfs destroy $vol
+		destroy_dataset $vol
 	done
 }
 log_onexit cleanup