netfilter: ipset: "Directory not empty" error message

When an entry flagged with "nomatch" was tested by ipset, it
returned the error message "Kernel error received:
Directory not empty" instead of "<element> is NOT in set <setname>"
(reported by John Brendler).

The internal error code was not properly transformed before returning
to userspace, fixed.
Signed-off-by: NJozsef Kadlecsik <kadlec@blackhole.kfki.hu>

net/netfilter/ipset/ip_set_core.c

@@ -1470,7 +1470,8 @@ ip_set_utest(struct sock *ctnl, struct sk_buff *skb,
 	if (ret == -EAGAIN)
 		ret = 1;
 
-	return ret < 0 ? ret : ret > 0 ? 0 : -IPSET_ERR_EXIST;
+	return (ret < 0 && ret != -ENOTEMPTY) ? ret :
+		ret > 0 ? 0 : -IPSET_ERR_EXIST;
 }
 
 /* Get headed data of a set */