macvlan: Avoid unnecessary multicast cloning

Currently we always queue a multicast packet for further processing,
even if none of the macvlan devices are subscribed to the address.

This patch optimises this by adding a global multicast filter for
a macvlan_port.

Note that this patch doesn't handle the broadcast addresses of the
individual macvlan devices correctly, if they are not all identical
to vlan->lowerdev.  However, this is already broken because there
is no mechanism in place to update the individual multicast filters
when you change the broadcast address.

If someone cares enough they should fix this by collecting all
broadcast addresses for a macvlan as we do for multicast and unicast.

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