batman-adv: remove references for global tt entries

struct tt_global_entry holds a reference to an orig_node which must be
decremented before deallocating the structure.

Signed-off-by: Simon Wunderlich <siwu@hrz.tu-chemnitz.de>
Tested-by: Alexey Fisher <bug-track@fisher-privat.net>
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>