netfilter: conntrack: consider ct netns in early_drop logic

When iterating, skip conntrack entries living in a different netns.

We could ignore netns and kill some other non-assured one, but it
has two problems:

- a netns can kill non-assured conntracks in other namespace
- we would start to 'over-subscribe' the affected/overlimit netns.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>