Adds support for Layer 2 multicast by implementing the DSA port_mdb_*
callbacks. The Kernel bridge listens to IGMP/MLD messages trapped to
the CPU-port, and calls the Multicast Forwarding Database updates.
The updates manage the L2 forwarding entries and the multicast
port-maps.
Signed-off-by: Birger Koblitz <git@birger-koblitz.de>
cde31976e3