xfrm: Use VRF master index if output device is enslaved

Directs route lookups to VRF table. Compiles out if NET_VRF is not
enabled. With this patch able to successfully bring up ipsec tunnels
in VRFs, even with duplicate network configuration.

Signed-off-by: David Ahern <dsa@cumulusnetworks.com>
Acked-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Acked-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: David S. Miller <davem@davemloft.net>