netfilter: netns nf_conntrack: GRE conntracking in netns

* make keymap list per-netns
* per-netns keymal lock (not strictly necessary)
* flush keymap at netns stop and module unload.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>
diff --git a/net/netfilter/nf_conntrack_pptp.c b/net/netfilter/nf_conntrack_pptp.c
index 5db7df5..e47d5de 100644
--- a/net/netfilter/nf_conntrack_pptp.c
+++ b/net/netfilter/nf_conntrack_pptp.c
@@ -602,7 +602,7 @@
 static void __exit nf_conntrack_pptp_fini(void)
 {
 	nf_conntrack_helper_unregister(&pptp);
-	nf_ct_gre_keymap_flush();
+	nf_ct_gre_keymap_flush(&init_net);
 }
 
 module_init(nf_conntrack_pptp_init);