There is one main difference in mscc_ocelot between IP multicast and L2
multicast. With IP multicast, destination ports are encoded into the
upper bytes of the multicast MAC address. Example: to deliver the
address 01:00:5E:11:22:33 to ports 3, 8, and 9, one would need to
program the address of 00:03:08:11:22:33 into hardware. Whereas for L2
multicast, the MAC table entry points to a Port Group ID (PGID), and
that PGID contains the port mask that the packet will be forwarded to.
As to why it is this way, no clue. My guess is that not all port
combinations can be supported simultaneously with the limited number of
PGIDs, and this was somehow an issue for IP multicast but not for L2
multicast. Anyway.

Prior to this change, the raw L2 multicast code was bogus, due to the
fact that there wasn't really any way to test it using the bridge code.
There were 2 issues:
- A multicast PGID was allocated for each MDB entry, but it wasn't in
  fact programmed to hardware. It was dummy.
- In fact we don't want to reserve a multicast PGID for every single MDB
  entry. That would be odd because we can only have ~60 PGIDs, but
  thousands of MDB entries. So instead, we want to reserve a multicast
  PGID for every single port combination for multicast traffic. And
  since we can have 2 (or more) MDB entries delivered to the same port
  group (and therefore PGID), we need to reference-count the PGIDs.
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>