net: dsa: execute dsa_switch_mdb_add only for routing port in cross-chip topologies

net: dsa: execute dsa_switch_mdb_add only for routing port in cross-chip topologies

Currently, the notifier for adding a multicast MAC address matches on
the targeted port and on all DSA links in the system, be they upstream
or downstream links.

This leads to a considerable amount of useless traffic.

Consider this daisy chain topology, and a MDB add notifier emitted on
sw0p0. It matches on sw0p0, sw0p3, sw1p3 and sw2p4.

   sw0p0     sw0p1     sw0p2     sw0p3     sw0p4
[  user ] [  user ] [  user ] [  dsa  ] [  cpu  ]
[   x   ] [       ] [       ] [   x   ] [       ]
                                  |
                                  +---------+
                                            |
   sw1p0     sw1p1     sw1p2     sw1p3     sw1p4
[  user ] [  user ] [  user ] [  dsa  ] [  dsa  ]
[       ] [       ] [       ] [   x   ] [   x   ]
                                  |
                                  +---------+
                                            |
   sw2p0     sw2p1     sw2p2     sw2p3     sw2p4
[  user ] [  user ] [  user ] [  user ] [  dsa  ]
[       ] [       ] [       ] [       ] [   x   ]

But switch 0 has no reason to send the multicast traffic for that MAC
address on sw0p3, which is how it reaches switches 1 and 2. Those
switches don't expect, according to the user configuration, to receive
this multicast address from switch 1, and they will drop it anyway,
because the only valid destination is the port they received it on.
They only need to configure themselves to deliver that multicast address
_towards_ switch 1, where the MDB entry is installed.

Similarly, switch 1 should not send this multicast traffic towards
sw1p3, because that is how it reaches switch 2.

With this change, the heat map for this MDB notifier changes as follows:

   sw0p0     sw0p1     sw0p2     sw0p3     sw0p4
[  user ] [  user ] [  user ] [  dsa  ] [  cpu  ]
[   x   ] [       ] [       ] [       ] [       ]
                                  |
                                  +---------+
                                            |
   sw1p0     sw1p1     sw1p2     sw1p3     sw1p4
[  user ] [  user ] [  user ] [  dsa  ] [  dsa  ]
[       ] [       ] [       ] [       ] [   x   ]
                                  |
                                  +---------+
                                            |
   sw2p0     sw2p1     sw2p2     sw2p3     sw2p4
[  user ] [  user ] [  user ] [  user ] [  dsa  ]
[       ] [       ] [       ] [       ] [   x   ]

Now the mdb notifier behaves the same as the fdb notifier.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>