ipvs: batch __ip_vs_cleanup

It's better to batch __ip_vs_cleanup to speedup ipvs
connections dismantle.

Signed-off-by: Haishuang Yan <yanhaishuang@cmss.chinamobile.com>
Acked-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Simon Horman <horms@verge.net.au>