xfrm: iptfs: add user packet (tunnel ingress) handling

Add tunnel packet output functionality. This is code handles
the ingress to the tunnel.

Signed-off-by: Christian Hopps <[email protected]>
Tested-by: Antony Antony <[email protected]>
Signed-off-by: Steffen Klassert <[email protected]>

Author: choppsv1