netfilter: nft_flow_offload: use direct xmit if hardware offload is enabled

If there is a forward path to reach an ethernet device and hardware
offload is enabled, then use the direct xmit path.

Moreover, store the real device in the direct xmit path info since
software datapath uses dev_hard_header() to push the layer encapsulation
headers while hardware offload refers to the real device.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>