net: cls_u32: fix error code for invalid flags

'err' variable is not set in this test, we would return whatever
previous test set 'err' to.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Acked-by: Sridhar Samudrala <sridhar.samudrala@intel.com>
Acked-by: John Fastabend <john.r.fastabend@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/cls_u32.c

@@ -863,7 +863,7 @@ static int u32_change(struct net *net, struct sk_buff *in_skb,
 	if (tb[TCA_U32_FLAGS]) {
 		flags = nla_get_u32(tb[TCA_U32_FLAGS]);
 		if (!tc_flags_valid(flags))
-			return err;
+			return -EINVAL;
 	}
 
 	n = (struct tc_u_knode *)*arg;