net: Convert diag_net_ops

These pernet operations just create and destroy netlink
socket. The socket is pernet and else operations don't
touch it.

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