net: Convert proto_gre_net_ops

These pernet_operations register and unregister sysctl.
nf_conntrack_l4proto_gre4->init_net is simple memory
initializer. Also, exit method removes gre keymap_list,
which is per-net. This looks safe to be executed
in parallel with other pernet_operations.

Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: David S. Miller <davem@davemloft.net>