igb: Add support for MAC address filters specifying source addresses

Makes it possible to direct packets to queues based on their source
address. Documents the expected usage of the 'flags' parameter.

Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>