Switch to from NF_CT_ASSERT to WARN_ON like kernel commit:

commit 44d6e2f27328b254111dd716fde45b3b59b8a4f7
Author: Varsha Rao <rvarsha016@gmail.com>
Date:   Wed Aug 30 13:37:11 2017 +0530

    net: Replace NF_CT_ASSERT() with WARN_ON().

diff -Nurp xtables-addons-2.13.orig/extensions/xt_DNETMAP.c xtables-addons-2.13/extensions/xt_DNETMAP.c
--- xtables-addons-2.13.orig/extensions/xt_DNETMAP.c	2017-06-29 15:46:37.037114880 +0300
+++ xtables-addons-2.13/extensions/xt_DNETMAP.c	2017-11-13 16:45:52.497499811 +0200
@@ -377,9 +377,9 @@ dnetmap_tg(struct sk_buff *skb, const st
 	unsigned int hooknum = par->hooknum;
 #endif
 
-	NF_CT_ASSERT(hooknum == NF_INET_POST_ROUTING ||
-		     hooknum == NF_INET_LOCAL_OUT ||
-		     hooknum == NF_INET_PRE_ROUTING);
+	WARN_ON(hooknum != NF_INET_POST_ROUTING ||
+		hooknum != NF_INET_LOCAL_OUT ||
+		hooknum != NF_INET_PRE_ROUTING);
 	ct = nf_ct_get(skb, &ctinfo);
 
 	jttl = tginfo->flags & XT_DNETMAP_TTL ? tginfo->ttl * HZ : jtimeout;