netfilter: conntrack: do not dump other netns's conntrack entries via proc

We should skip the conntracks that belong to a different namespace,
otherwise other unrelated netns's conntrack entries will be dumped via
/proc/net/nf_conntrack.

Fixes: 56d52d48 ("netfilter: conntrack: use a single hashtable for all namespaces")
Signed-off-by: Liping Zhang <liping.zhang@spreadtrum.com>
Reviewed-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>