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 <adobriyan@gmail.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>