batman-adv: purge bridge loop avoidance when its disabled

When bridge loop avoidance is disabled through sysfs, the internal
datastructures are not disabled, but only BLA operations are disabled.
To be sure that they are removed, purge the data immediately. That is
especially useful if a firmwares network state is changed, and the BLA
wait periods should restart on the new network.

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>