net: dsa: include fdb entries pointing to bridge in the host fdb list

The bridge supports a legacy way of adding local (non-forwarded) FDB
entries, which works on an individual port basis:

bridge fdb add dev swp0 00:01:02:03:04:05 master local

As well as a new way, added by Roopa Prabhu in commit 3741873b
("bridge: allow adding of fdb entries pointing to the bridge device"):

bridge fdb add dev br0 00:01:02:03:04:05 self local

The two commands are functionally equivalent, except that the first one
produces an entry with fdb->dst == swp0, and the other an entry with
fdb->dst == NULL. The confusing part, though, is that even if fdb->dst
is swp0 for the 'local on port' entry, that destination is not used.

Nonetheless, the idea is that the bridge has reference counting for
local entries, and local entries pointing towards the bridge are still
'as local' as local entries for a port.

The bridge adds the MAC addresses of the interfaces automatically as
FDB entries with is_local=1. For the MAC address of the ports, fdb->dst
will be equal to the port, and for the MAC address of the bridge,
fdb->dst will point towards the bridge (i.e. be NULL). Therefore, if the
MAC address of the bridge is not inherited from either of the physical
ports, then we must explicitly catch local FDB entries emitted towards
the br0, otherwise we'll miss the MAC address of the bridge (and, of
course, any entry with 'bridge add dev br0 ... self local').

Co-developed-by: Tobias Waldekranz <tobias@waldekranz.com>
Signed-off-by: Tobias Waldekranz <tobias@waldekranz.com>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>