netfilter: nat: destroy nat mappings on module exit path only

We don't need pernetns cleanup anymore.  If the netns is being
destroyed, conntrack netns exit will kill all entries in this namespace,
and neither conntrack hash table nor bysource hash are per namespace.

For the rmmod case, we have to make sure we remove all entries from the
nat bysource table, so call the new nf_ct_iterate_destroy in module exit
path.

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