net/xfrm: IPsec tunnel mode fix inner_ipproto setting in sec_path

The inner_ipproto saves the inner IP protocol of the plain
text packet. This allows vendor's IPsec feature making offload
decision at skb's features_check and configuring hardware at
ndo_start_xmit, current code implenetation did not handle the
case where IPsec is used in tunnel mode.

Fix by handling the case when IPsec is used in tunnel mode by
reading the protocol of the plain text packet IP protocol.

Fixes: fa453523 ("net/xfrm: Add inner_ipproto into sec_path")
Signed-off-by: Raed Salem <raeds@nvidia.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>