netfilter: flowtable: fix nft_flow_route source address for nat case

For snat and dnat cases, the saddr should be taken from reverse tuple.

Fixes: 3412e164 (netfilter: flowtable: nft_flow_route use more data for reverse route)
Signed-off-by: wenxu <wenxu@chinatelecom.cn>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>