macvlan: Add netlink attribute for broadcast cutoff

Make the broadcast cutoff configurable through netlink.  Note
that macvlan is weird because there is no central device for
us to configure (the lowerdev could be anything).  So all the
options are duplicated over what could be thousands of child
devices.

IFLA_MACVLAN_BC_QUEUE_LEN took the approach of taking the maximum
of all child device settings.  This is unnecessary as we could
simply store the option in the port device and take the last
child device that gets updated as the value to use.

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