batman-adv: Add fragmentation mesh genl configuration

The mesh interface can fragment unicast packets when the packet size
exceeds the outgoing slave/hard-interface MTU.

The BATADV_CMD_SET_MESH/BATADV_CMD_GET_MESH commands allow to set/get the
configuration of this feature using the BATADV_ATTR_FRAGMENTATION_ENABLED
attribute. Setting the u8 to zero will disable this feature and setting it
to something else is enabling this feature.

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