md: raid0: fix error return from create_stripe_zones.

Create_stripe_zones returns an error slightly differently to
raid0_run and to raid0_takeover_*.

The error returned used by the second was wrong and an error would
result in mddev->private being set to NULL and sooner or later a
crash.

So never return NULL, return ERR_PTR(err), not NULL from
create_stripe_zones.

This bug has been present since 2.6.35 so the fix is suitable
for any kernel since then.

Cc: stable@vger.kernel.org
Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/raid0.c

@@ -289,7 +289,7 @@ static int create_strip_zones(struct mddev *mddev, struct r0conf **private_conf)
 	kfree(conf->strip_zone);
 	kfree(conf->devlist);
 	kfree(conf);
-	*private_conf = NULL;
+	*private_conf = ERR_PTR(err);
 	return err;
 }