netfilter: ctnetlink: support CTA_FILTER for flush

From cb8aa9a3, we can use kernel side filtering for dump, but
this capability is not available for flush.

This Patch allows advanced filter with CTA_FILTER for flush

Performace
1048576 ct flows in total, delete 50,000 flows by origin src ip
3.06s -> dump all, compare and delete
584ms -> directly flush with filter

Signed-off-by: Changliang Wu <changliang.wu@smartx.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>