batman-adv: iv_ogm, Reduce code duplication

The difference between tq1 and tq2 are calculated the same way in two
separate functions.

This patch moves the common code to a separate function
'batadv_iv_ogm_neigh_diff' which handles everything necessary. The other
two functions can then handle errors and use the difference directly.

Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
[sven@narfation.org: rebased on current version, initialize return variable
in batadv_iv_ogm_neigh_diff, add kerneldoc, convert to bool return type]
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>