ZTS: refreserv_005_pos.ksh

When recursively destroying the dataset it's possible for the
dataset volume to be open by an unrelated process, like blkid.
Use the destroy_dataset() which will retry when this occurs.

Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #10305

tests/zfs-tests/tests/functional/refreserv/refreserv_005_pos.ksh

@@ -45,9 +45,9 @@ verify_runnable "global"
 
 function cleanup
 {
-	log_must zfs destroy -rf $TESTPOOL/$TESTFS
-	log_must zfs create $TESTPOOL/$TESTFS
-	log_must zfs set mountpoint=$TESTDIR $TESTPOOL/$TESTFS
+	destroy_dataset "$fs" "-rf"
+	log_must zfs create $fs
+	log_must zfs set mountpoint=$TESTDIR $fs
 }
 
 log_assert "Volume (ref)reservation is not limited by volsize"