xfrm: prevent configuration of interface index when offload is used

xfrm: prevent configuration of interface index when offload is used

Both packet and crypto offloads perform decryption while packet is
arriving to the HW from the wire. It means that there is no possible
way to perform lookup on XFRM if_id as it can't be set to be "before' HW.

So instead of silently ignore this configuration, let's warn users about
misconfiguration.

Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>