netfilter: nf_nat: place conntrack in source hash after SNAT is done

If SNAT isn't done, the wrong info maybe got by the other cts.

As the filter table is after DNAT table, the packets dropped in filter
table also bother bysource hash table.

Signed-off-by: Changli Gao <xiaosuo@gmail.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>