net: dsa: reference count the FDB addresses at the cross-chip notifier level

The same concerns expressed for host MDB entries are valid for host FDBs
just as well:

- in the case of multiple bridges spanning the same switch chip, deleting
  a host FDB entry that belongs to one bridge will result in breakage to
  the other bridge
- not deleting FDB entries across DSA links means that the switch's
  hardware tables will eventually run out, given enough wear&tear

So do the same thing and introduce reference counting for CPU ports and
DSA links using the same data structures as we have for MDB entries.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/dsa.h

@@ -288,6 +288,7 @@ struct dsa_port {
 	/* List of MAC addresses that must be forwarded on this port.
 	 * These are only valid on CPU ports and DSA links.
 	 */
+	struct list_head	fdbs;
 	struct list_head	mdbs;
 
 	bool setup;