Staging: batman-adv: fragment forwarded packets

If a packet is too big to be forwarded over an interface it will be
fragmented on-the-fly (if fragmentation is enabled).

Signed-off-by: Andreas Langer <an.langer@gmx.de>
Signed-off-by: Sven Eckelmann <sven.eckelmann@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>