net: Use VRF index for oif in ip_send_unicast_reply

If output device is not specified use VRF device if input device is
enslaved. This is needed to ensure tcp acks and resets go out VRF device.

Signed-off-by: David Ahern <dsa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>