ZTS: Fix zfs_create_013_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 #7847

tests/zfs-tests/tests/functional/cli_root/zfs_create/zfs_create_013_pos.ksh

@@ -48,8 +48,7 @@ function cleanup
 {
 	typeset -i j=0
 	while [[ $j -lt ${#size[*]} ]]; do
-		datasetexists $TESTPOOL/${LONGFSNAME}${size[j]} && \
-		    log_must zfs destroy $TESTPOOL/${LONGFSNAME}${size[j]}
+		destroy_dataset $TESTPOOL/${LONGFSNAME}${size[j]}
 		((j = j + 1))
 	done
 }