xfrm: Prevent locally generated packets from direct output in tunnel mode

xfrm: Prevent locally generated packets from direct output in tunnel mode

Add a check to ensure locally generated packets (skb->sk != NULL) do
not use direct output in tunnel mode, as these packets require proper
L2 header setup that is handled by the normal XFRM processing path.

Fixes: 5eddd76ec2fd ("xfrm: fix tunnel mode TX datapath in packet offload mode")
Signed-off-by: Jianbo Liu <jianbol@nvidia.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>