netfilter: ctnetlink: cleanup conntrack update preliminary checkings

This patch moves the preliminary checkings that must be fulfilled
to update a conntrack, which are the following:

 * NAT manglings cannot be updated
 * Changing the master conntrack is not allowed.

This patch is a cleanup.
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Patrick McHardy <kaber@trash.net>

net/netfilter/nf_conntrack_netlink.c

@@ -1062,6 +1062,10 @@ ctnetlink_change_conntrack(struct nf_conn *ct, struct nlattr *cda[])
 {
 	int err;
 
+	/* only allow NAT changes and master assignation for new conntracks */
+	if (cda[CTA_NAT_SRC] || cda[CTA_NAT_DST] || cda[CTA_TUPLE_MASTER])
+		return -EOPNOTSUPP;
+
 	if (cda[CTA_HELP]) {
 		err = ctnetlink_change_helper(ct, cda);
 		if (err < 0)
@@ -1323,17 +1327,6 @@ ctnetlink_new_conntrack(struct sock *ctnl, struct sk_buff *skb,
 	if (!(nlh->nlmsg_flags & NLM_F_EXCL)) {
 		struct nf_conn *ct = nf_ct_tuplehash_to_ctrack(h);
 
-		/* we only allow nat config for new conntracks */
-		if (cda[CTA_NAT_SRC] || cda[CTA_NAT_DST]) {
-			err = -EOPNOTSUPP;
-			goto out_unlock;
-		}
-		/* can't link an existing conntrack to a master */
-		if (cda[CTA_TUPLE_MASTER]) {
-			err = -EOPNOTSUPP;
-			goto out_unlock;
-		}
-
 		err = ctnetlink_change_conntrack(ct, cda);
 		if (err == 0) {
 			nf_conntrack_get(&ct->ct_general);