net/sched: cls_flower: Properly handle classifier flags dumping

Dump the classifier flags only if non zero and make sure to check
the return status of the handler that puts them into the netlink msg.
Signed-off-by: NOr Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/sched/cls_flower.c

@@ -1229,7 +1229,8 @@ static int fl_dump(struct net *net, struct tcf_proto *tp, unsigned long fh,
 	if (fl_dump_key_flags(skb, key->control.flags, mask->control.flags))
 		goto nla_put_failure;
 
-	nla_put_u32(skb, TCA_FLOWER_FLAGS, f->flags);
+	if (f->flags && nla_put_u32(skb, TCA_FLOWER_FLAGS, f->flags))
+		goto nla_put_failure;
 
 	if (tcf_exts_dump(skb, &f->exts))
 		goto nla_put_failure;