netfilter: flowtable: use dev_fill_forward_path() to obtain ingress device

Obtain the ingress device in the tuple from the route in the reply
direction. Use dev_fill_forward_path() instead to get the real ingress
device for this flow.

Fall back to use the ingress device that the IP forwarding route
provides if:

- dev_fill_forward_path() finds no real ingress device.
- the ingress device that is obtained is not part of the flowtable
  devices.
- this route has a xfrm policy.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>