batman-adv: Add multicast forceflood mesh genl configuration

The mesh interface can optimize the flooding of multicast packets based on
the content of the global translation tables. To disable this behavior and
use the broadcast-like flooding of the packets, forceflood has to be
enabled.

The BATADV_CMD_SET_MESH/BATADV_CMD_GET_MESH commands allow to set/get the
configuration of this feature using the
BATADV_ATTR_MULTICAST_FORCEFLOOD_ENABLED attribute. Setting the u8 to zero
will disable this feature (allowing multicast optimizations) and setting it
to something else is enabling this feature (forcing simple flooding).

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>