iplink:macvlan: Added bcqueuelen parameter

iplink:macvlan: Added bcqueuelen parameter

This patch allows the user to set and retrieve the
IFLA_MACVLAN_BC_QUEUE_LEN parameter via the bcqueuelen
command line argument

This parameter controls the requested size of the queue for
broadcast and multicast packages in the macvlan driver.

If not specified, the driver default (1000) will be used.

Note: The request is per macvlan but the actually used queue
length per port is the maximum of any request to any macvlan
connected to the same port.

For this reason, the used queue length IFLA_MACVLAN_BC_QUEUE_LEN_USED
is also retrieved and displayed in order to aid in the understanding
of the setting. However, it can of course not be directly set.

Signed-off-by: Thomas Karlsson <thomas.karlsson@paneda.se>
Signed-off-by: David Ahern <dsahern@gmail.com>