macvlan: implement bridge, VEPA and private mode

This allows each macvlan slave device to be in one
of three modes, depending on the use case:

MACVLAN_PRIVATE:
  The device never communicates with any other device
  on the same upper_dev. This even includes frames
  coming back from a reflective relay, where supported
  by the adjacent bridge.

MACVLAN_VEPA:
  The new Virtual Ethernet Port Aggregator (VEPA) mode,
  we assume that the adjacent bridge returns all frames
  where both source and destination are local to the
  macvlan port, i.e. the bridge is set up as a reflective
  relay.
  Broadcast frames coming in from the upper_dev get
  flooded to all macvlan interfaces in VEPA mode.
  We never deliver any frames locally.

MACVLAN_BRIDGE:
  We provide the behavior of a simple bridge between
  different macvlan interfaces on the same port. Frames
  from one interface to another one get delivered directly
  and are not sent out externally. Broadcast frames get
  flooded to all other bridge ports and to the external
  interface, but when they come back from a reflective
  relay, we don't deliver them again.
  Since we know all the MAC addresses, the macvlan bridge
  mode does not require learning or STP like the bridge
  module does.

Based on an earlier patch "macvlan: Reflect macvlan packets
meant for other macvlan devices" by Eric Biederman.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Patrick McHardy <kaber@trash.net>
Cc: Eric Biederman <ebiederm@xmission.com>
Signed-off-by: David S. Miller <davem@davemloft.net>