Introduce a brother of br_vlan_get_info() which is protected by the RCU
mechanism, as opposed to br_vlan_get_info() which relies on taking the
write-side rtnl_mutex.

This is needed for drivers which need to find out whether a bridge port
has a VLAN configured or not. For example, certain DSA switches might
not offer complete source port identification to the CPU on RX, just the
VLAN in which the packet was received. Based on this VLAN, we cannot set
an accurate skb->dev ingress port, but at least we can configure one
that behaves the same as the correct one would (this is possible because
DSA sets skb->offload_fwd_mark = 1).

When we look at the bridge RX handler (br_handle_frame), we see that
what matters regarding skb->dev is the VLAN ID and the port STP state.
So we need to select an skb->dev that has the same bridge VLAN as the
packet we're receiving, and is in the LEARNING or FORWARDING STP state.
The latter is easy, but for the former, we should somehow keep a shadow
list of the bridge VLANs on each port, and a lookup table between VLAN
ID and the 'designated port for imprecise RX'. That is rather
complicated to keep in sync properly (the designated port per VLAN needs
to be updated on the addition and removal of a VLAN, as well as on the
join/leave events of the bridge on that port).

So, to avoid all that complexity, let's just iterate through our finite
number of ports and ask the bridge, for each packet: "do you have this
VLAN configured on this port?".

Cc: Roopa Prabhu <roopa@nvidia.com>
Cc: Nikolay Aleksandrov <nikolay@nvidia.com>
Cc: Ido Schimmel <idosch@nvidia.com>
Cc: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>