batman-adv: implement batadv_tt_entries

Implement batadv_tt_entries() to get the number of entries
fitting in a given amount of bytes. This computation is done
several times in the code and therefore it is useful to have
an helper function.

Signed-off-by: Antonio Quartulli <antonio@open-mesh.com>
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>