netfilter: nf_conntrack_proto_gre: switch to register_pernet_gen_subsys()

register_pernet_gen_device() can't be used is nf_conntrack_pptp module is
also used (compiled in or loaded).

Right now, proto_gre_net_exit() is called before nf_conntrack_pptp_net_exit().
The former shutdowns and frees GRE piece of netns, however the latter
absolutely needs it to flush keymap. Oops is inevitable.

Switch to shiny new register_pernet_gen_subsys() to get correct ordering in
netns ops list.

Signed-off-by: Alexey Dobriyan <[e-mail]>
Signed-off-by: Patrick McHardy <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/netfilter/nf_conntrack_proto_gre.c

···
 	rv = nf_conntrack_l4proto_register(&nf_conntrack_l4proto_gre4);
 	if (rv < 0)
 		return rv;
-	rv = register_pernet_gen_device(&proto_gre_net_id, &proto_gre_net_ops);
+	rv = register_pernet_gen_subsys(&proto_gre_net_id, &proto_gre_net_ops);
 	if (rv < 0)
 		nf_conntrack_l4proto_unregister(&nf_conntrack_l4proto_gre4);
 	return rv;
···
 static void nf_ct_proto_gre_fini(void)
 {
 	nf_conntrack_l4proto_unregister(&nf_conntrack_l4proto_gre4);
-	unregister_pernet_gen_device(proto_gre_net_id, &proto_gre_net_ops);
+	unregister_pernet_gen_subsys(proto_gre_net_id, &proto_gre_net_ops);
 }
 
 module_init(nf_ct_proto_gre_init);