[NETFILTER]: Fix DELETE_LIST oopses.

We've now narrowed down the issue of kernel oopses in combination with
'LIST_DELETE' syslog messages happening in certain setups.

Apparently people who do not enable CONFIG_IP_NF_NAT_LOCAL and do
DNAT/REDIRECT and want to connect locally from the gateway via DNAT to
the DNAT'ed address experience the bug ;)

Patch courtesy of KOVACS Krisztian and Henrik Nordstrom

net/ipv4/netfilter/ip_nat_standalone.c

@@ -124,7 +124,16 @@ ip_nat_fn(unsigned int hooknum,
 		WRITE_LOCK(&ip_nat_lock);
 		/* Seen it before?  This can happen for loopback, retrans,
 		   or local packets.. */
-		if (!(info->initialized & (1 << maniptype))) {
+		if (!(info->initialized & (1 << maniptype))
+#ifndef CONFIG_IP_NF_NAT_LOCAL
+		    /* If this session has already been confirmed we must not
+		     * touch it again even if there is no mapping set up.
+		     * Can only happen on local->local traffic with
+		     * CONFIG_IP_NF_NAT_LOCAL disabled.
+		     */
+		    && !(ct->status & IPS_CONFIRMED)
+#endif
+		    ) {
 			unsigned int ret;
 
 			if (ct->master