net: sched: fix flush on non-existing chain

User was able to perform filter flush on chain 0 even if it didn't have
any filters in it. With the patch that avoided implicit chain 0
creation, this changed. So in case user wants filter flush on chain
which does not exist, just return success. There's no reason for non-0
chains to behave differently than chain 0, so do the same for them.

Reported-by: Ido Schimmel <idosch@mellanox.com>
Fixes: f71e0ca4db18 ("net: sched: Avoid implicit chain 0 creation")
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/cls_api.c

@@ -1389,6 +1389,13 @@ static int tc_del_tfilter(struct sk_buff *skb, struct nlmsghdr *n,
 	}
 	chain = tcf_chain_get(block, chain_index, false);
 	if (!chain) {
+		/* User requested flush on non-existent chain. Nothing to do,
+		 * so just return success.
+		 */
+		if (prio == 0) {
+			err = 0;
+			goto errout;
+		}
 		NL_SET_ERR_MSG(extack, "Cannot find specified filter chain");
 		err = -EINVAL;
 		goto errout;