batman-adv: only call post function if something changed

Currently, the post function is also called on errors or if there were
no changes, which is redundant for the functions currently using these
facilities.

Signed-off-by: Simon Wunderlich <simon@open-mesh.com>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Antonio Quartulli <a@unstable.cc>