macvlan: Skip broadcast queue if multicast with single receiver

As it stands all broadcast and multicast packets are queued and
processed in a work queue.  This is so that we don't overwhelm
the receive softirq path by generating thousands of packets or
more (see commit 412ca155 "macvlan: Move broadcasts into a
work queue").

As such all multicast packets will be delayed, even if they will
be received by a single macvlan device.  As using a workqueue
is not free in terms of latency, we should avoid this where possible.

This patch adds a new filter to determine which addresses should
be delayed and which ones won't.  This is done using a crude
counter of how many times an address has been added to the macvlan
port (ha->synced).  For now if an address has been added more than
once, then it will be considered to be broadcast.  This could be
tuned further by making this threshold configurable.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>