net: Convert sctp_ctrlsock_ops

These pernet_operations create and destroy net::sctp::ctl_sock.
Since pernet_operations do not send sctp packets each other,
they look safe to be marked as async.

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