rocker: add offload_fwd_mark support

If device flags ingress packet as "fwd offload", mark the
skb->offlaod_fwd_mark using the ingress port's dev->offlaod_fwd_mark.  This
will be the hint to the kernel that this packet has already been forwarded
by device to egress ports matching skb->offlaod_fwd_mark.

For rocker, derive port dev->offlaod_fwd_mark based on device switch ID and
port ifindex.  If port is bridged, use the bridge ifindex rather than the
port ifindex.

Signed-off-by: Scott Feldman <sfeldma@gmail.com>
Acked-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: David S. Miller <davem@davemloft.net>