ipvs: add a 'default' case in do_ip_vs_set_ctl()

It is better to return the default switch case with
'-EINVAL', in case new commands are added. otherwise,
return a uninitialized value of ret.

Signed-off-by: Li Qiong <liqiong@nfschina.com>
Reviewed-by: Simon Horman <horms@verge.net.au>
Acked-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>