netfilter: xtables: fix mangle tables

In POST_ROUTING hook, calling dev_net(in) is going to oops.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>
diff --git a/net/ipv4/netfilter/iptable_mangle.c b/net/ipv4/netfilter/iptable_mangle.c
index 58d7097..c833330 100644
--- a/net/ipv4/netfilter/iptable_mangle.c
+++ b/net/ipv4/netfilter/iptable_mangle.c
@@ -87,7 +87,9 @@
 {
 	if (hook == NF_INET_LOCAL_OUT)
 		return ipt_local_hook(hook, skb, in, out, okfn);
-
+	if (hook == NF_INET_POST_ROUTING)
+		return ipt_do_table(skb, hook, in, out,
+				    dev_net(out)->ipv4.iptable_mangle);
 	/* PREROUTING/INPUT/FORWARD: */
 	return ipt_do_table(skb, hook, in, out,
 			    dev_net(in)->ipv4.iptable_mangle);