net: bridge: disambiguate offload_fwd_mark

Before this change, four related - but distinct - concepts where named
offload_fwd_mark:

- skb->offload_fwd_mark: Set by the switchdev driver if the underlying
  hardware has already forwarded this frame to the other ports in the
  same hardware domain.

- nbp->offload_fwd_mark: An idetifier used to group ports that share
  the same hardware forwarding domain.

- br->offload_fwd_mark: Counter used to make sure that unique IDs are
  used in cases where a bridge contains ports from multiple hardware
  domains.

- skb->cb->offload_fwd_mark: The hardware domain on which the frame
  ingressed and was forwarded.

Introduce the term "hardware forwarding domain" ("hwdom") in the
bridge to denote a set of ports with the following property:

    If an skb with skb->offload_fwd_mark set, is received on a port
    belonging to hwdom N, that frame has already been forwarded to all
    other ports in hwdom N.

By decoupling the name from "offload_fwd_mark", we can extend the
term's definition in the future - e.g. to add constraints that
describe expected egress behavior - without overloading the meaning of
"offload_fwd_mark".

- nbp->offload_fwd_mark thus becomes nbp->hwdom.

- br->offload_fwd_mark becomes br->last_hwdom.

- skb->cb->offload_fwd_mark becomes skb->cb->src_hwdom. The slight
  change in naming here mandates a slight change in behavior of the
  nbp_switchdev_frame_mark() function. Previously, it only set this
  value in skb->cb for packets with skb->offload_fwd_mark true (ones
  which were forwarded in hardware). Whereas now we always track the
  incoming hwdom for all packets coming from a switchdev (even for the
  packets which weren't forwarded in hardware, such as STP BPDUs, IGMP
  reports etc). As all uses of skb->cb->offload_fwd_mark were already
  gated behind checks of skb->offload_fwd_mark, this will not introduce
  any functional change, but it paves the way for future changes where
  the ingressing hwdom must be known for frames coming from a switchdev
  regardless of whether they were forwarded in hardware or not
  (basically, if the skb comes from a switchdev, skb->cb->src_hwdom now
  always tracks which one).

  A typical example where this is relevant: the switchdev has a fixed
  configuration to trap STP BPDUs, but STP is not running on the bridge
  and the group_fwd_mask allows them to be forwarded. Say we have this
  setup:

        br0
       / | \
      /  |  \
  swp0 swp1 swp2

  A BPDU comes in on swp0 and is trapped to the CPU; the driver does not
  set skb->offload_fwd_mark. The bridge determines that the frame should
  be forwarded to swp{1,2}. It is imperative that forward offloading is
  _not_ allowed in this case, as the source hwdom is already "poisoned".

  Recording the source hwdom allows this case to be handled properly.

v2->v3: added code comments
v3->v6: none

Signed-off-by: Tobias Waldekranz <tobias@waldekranz.com>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Grygorii Strashko <grygorii.strashko@ti.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>