netfilter: flowtable: use dev_fill_forward_path() to obtain egress device

The egress device in the tuple is obtained from route. Use
dev_fill_forward_path() instead to provide the real egress device for
this flow whenever this is available.

The new FLOW_OFFLOAD_XMIT_DIRECT type uses dev_queue_xmit() to transmit
ethernet frames. Cache the source and destination hardware address to
use dev_queue_xmit() to transfer packets.

The FLOW_OFFLOAD_XMIT_DIRECT replaces FLOW_OFFLOAD_XMIT_NEIGH if
dev_fill_forward_path() finds a direct transmit path.

In case of topology updates, if peer is moved to different bridge port,
the connection will time out, reconnect will result in a new entry with
the correct path. Snooping fdb updates would allow for cleaning up stale
flowtable entries.

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