ipvs: Pass ipvs not net to register_ip_vs_app and unregister_ip_vs_app

Also move the tests for net_ipvs being NULL into __ip_vs_ftp_init
and __ip_vs_ftp_exit.  The only places where they possibly make
sense.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Acked-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Simon Horman <horms@verge.net.au>