net: Convert ip_vs_ftp_ops

These pernet_operations register and unregister ipvs app.
register_ip_vs_app(), unregister_ip_vs_app() and
register_ip_vs_app_inc() modify per-net structures,
and there are no global structures touched. So,
this looks safe to be marked as async.

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