xfrm: iptfs: add basic receive packet (tunnel egress) handling

Add handling of packets received from the tunnel. This implements
tunnel egress functionality.

Signed-off-by: Christian Hopps <chopps@labn.net>
Tested-by: Antony Antony <antony.antony@secunet.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>