netfilter: conntrack: use a single nat bysource table for all namespaces

We already include netns address in the hash, so we only need to use
net_eq in find_appropriate_src and can then put all entries into
same table.

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