net: dsa: fix db type confusion in host fdb/mdb add/del

We have the following code paths:

Host FDB (unicast RX filtering):

dsa_port_standalone_host_fdb_add()   dsa_port_bridge_host_fdb_add()
               |                                     |
               +--------------+         +------------+
                              |         |
                              v         v
                         dsa_port_host_fdb_add()

dsa_port_standalone_host_fdb_del()   dsa_port_bridge_host_fdb_del()
               |                                     |
               +--------------+         +------------+
                              |         |
                              v         v
                         dsa_port_host_fdb_del()

Host MDB (multicast RX filtering):

dsa_port_standalone_host_mdb_add()   dsa_port_bridge_host_mdb_add()
               |                                     |
               +--------------+         +------------+
                              |         |
                              v         v
                         dsa_port_host_mdb_add()

dsa_port_standalone_host_mdb_del()   dsa_port_bridge_host_mdb_del()
               |                                     |
               +--------------+         +------------+
                              |         |
                              v         v
                         dsa_port_host_mdb_del()

The logic added by commit 5e8a1e03 ("net: dsa: install secondary
unicast and multicast addresses as host FDB/MDB") zeroes out
db.bridge.num if the switch doesn't support ds->fdb_isolation
(the majority doesn't). This is done for a reason explained in commit
c2693363

 ("net: dsa: request drivers to perform FDB isolation").

Taking a single code path as example - dsa_port_host_fdb_add() - the
others are similar - the problem is that this function handles:
- DSA_DB_PORT databases, when called from
  dsa_port_standalone_host_fdb_add()
- DSA_DB_BRIDGE databases, when called from
  dsa_port_bridge_host_fdb_add()

So, if dsa_port_host_fdb_add() were to make any change on the
"bridge.num" attribute of the database, this would only be correct for a
DSA_DB_BRIDGE, and a type confusion for a DSA_DB_PORT bridge.

However, this bug is without consequences, for 2 reasons:

- dsa_port_standalone_host_fdb_add() is only called from code which is
  (in)directly guarded by dsa_switch_supports_uc_filtering(ds), and that
  function only returns true if ds->fdb_isolation is set. So, the code
  only executed for DSA_DB_BRIDGE databases.

- Even if the code was not dead for DSA_DB_PORT, we have the following
  memory layout:

struct dsa_bridge {
	struct net_device *dev;
	unsigned int num;
	bool tx_fwd_offload;
	refcount_t refcount;
};

struct dsa_db {
	enum dsa_db_type type;

	union {
		const struct dsa_port *dp; // DSA_DB_PORT
		struct dsa_lag lag;
		struct dsa_bridge bridge; // DSA_DB_BRIDGE
	};
};

So, the zeroization of dsa_db :: bridge :: num on a dsa_db structure of
type DSA_DB_PORT would access memory which is unused, because we only
use dsa_db :: dp for DSA_DB_PORT, and this is mapped at the same address
with dsa_db :: dev for DSA_DB_BRIDGE, thanks to the union definition.

It is correct to fix up dsa_db :: bridge :: num only from code paths
that come from the bridge / switchdev, so move these there.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20230329133819.697642-1-vladimir.oltean@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>