netfilter: ctnetlink: use EOPNOTSUPP instead of EINVAL if the conntrack	has no helper

This patch changes the return value if the conntrack has no helper assigned.
Instead of EINVAL, which is reserved for malformed messages, it returns
EOPNOTSUPP.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Patrick McHardy <kaber@trash.net>
diff --git a/net/netfilter/nf_conntrack_netlink.c b/net/netfilter/nf_conntrack_netlink.c
index 80a3f13..d87a939 100644
--- a/net/netfilter/nf_conntrack_netlink.c
+++ b/net/netfilter/nf_conntrack_netlink.c
@@ -1652,7 +1652,7 @@
 
 	if (!help || !help->helper) {
 		/* such conntrack hasn't got any helper, abort */
-		err = -EINVAL;
+		err = -EOPNOTSUPP;
 		goto out;
 	}