Staging: batman-adv: restructure fragmentation to handle batman unicast packets

The unicast_frag_send_skb() function expected 'raw' packets (without any
batman-adv header) to fragment them. This needs to be changed, so that
this function is able to fragment packets that already traveled inside
the mesh but need to be fragmented now.

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>