net: dsa: enable flooding for bridge ports

Switches work by learning the MAC address for each attached station by
monitoring traffic from each station.  When a station sends a packet,
the switch records which port the MAC address is connected to.

With IPv4 networking, before communication commences with a neighbour,
an ARP packet is broadcasted to all stations asking for the MAC address
corresponding with the IPv4.  The desired station responds with an ARP
reply, and the ARP reply causes the switch to learn which port the
station is connected to.

With IPv6 networking, the situation is rather different.  Rather than
broadcasting ARP packets, a "neighbour solicitation" is multicasted
rather than broadcasted.  This multicast needs to reach the intended
station in order for the neighbour to be discovered.

Once a neighbour has been discovered, and entered into the sending
stations neighbour cache, communication can restart at a point later
without sending a new neighbour solicitation, even if the entry in
the neighbour cache is marked as stale.  This can be after the MAC
address has expired from the forwarding cache of the DSA switch -
when that occurs, there is a long pause in communication.

Our DSA implementation for mv88e6xxx switches disables flooding of
multicast and unicast frames for bridged ports.  As per the above
description, this is fine for IPv4 networking, since the broadcasted
ARP queries will be sent to and received by all stations on the same
network.  However, this breaks IPv6 very badly - blocking neighbour
solicitations and later causing connections to stall.

The defaults that the Linux bridge code expect from bridges are for
unknown unicast and unknown multicast frames to be flooded to all ports
on the bridge, which is at odds to the defaults adopted by our DSA
implementation for mv88e6xxx switches.

This commit enables by default flooding of both unknown unicast and
unknown multicast frames whenever a port is added to a bridge, and
disables the flooding when a port leaves the bridge.  This means that
mv88e6xxx DSA switches now behave as per the bridge(8) man page, and
IPv6 works flawlessly through such a switch.

Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Vivien Didelot <vivien.didelot@gmail.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>