net: Convert xfrm_user_net_ops

These pernet_operations create and destroy net::xfrm::nlsk
socket of NETLINK_XFRM. There is only entry point, where
it's dereferenced, it's xfrm_user_rcv_msg(). There is no
in-kernel senders to this socket.

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