[NETFILTER]: nfctnetlink: Don't allow to change helper

There is no realistic situation to change helper (Who wants IRC helper to
track FTP traffic ?). Moreover, if we want to do that, we need to fix race
issue by nfctnetlink and running helper. That will add overhead to packet
processing. It wouldn't pay. So this rejects the request to change
helper. The requests to add or remove helper are accepted as ever.
Signed-off-by: NYasuyuki Kozakai <yasuyuki.kozakai@toshiba.co.jp>
Signed-off-by: NPatrick McHardy <kaber@trash.net>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/netfilter/nf_conntrack_netlink.c

@@ -869,8 +869,7 @@ ctnetlink_change_helper(struct nf_conn *ct, struct nfattr *cda[])
 		return 0;
 
 	if (help->helper)
-		/* we had a helper before ... */
-		nf_ct_remove_expectations(ct);
+		return -EBUSY;
 
 	/* need to zero data of old helper */
 	memset(&help->help, 0, sizeof(help->help));