batman-adv: move ring_buffer helper functions in bat_iv_ogm

the two lonely ring_buffer helper functions are used by the
bat_iv_ogm module only and therefore they can be moved
inside it.

Reported-by: Marek Lindner <lindner_marek@yahoo.de>
Signed-off-by: Antonio Quartulli <ordex@autistici.org>
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>