netfilter: ctnetlink: fix filtering for zone 0

previously filtering for the default zone would actually skip the zone
filter and flush all zones.

Fixes: eff3c558

 ("netfilter: ctnetlink: support filtering by zone")
Reported-by: Ilya Maximets <i.maximets@ovn.org>
Closes: https://lore.kernel.org/netdev/2032238f-31ac-4106-8f22-522e76df5a12@ovn.org/
Signed-off-by: Felix Huettner <felix.huettner@mail.schwarz>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>