batman-adv: Remove bat_ prefix from bat_{debugfs, sysfs}.{c, h}

The "bat_" prefix in the source files implementing the batman-adv sysfs and
debugfs interface doesn't have a special meaning and are only used by these
files and files that implement the actual B.A.T.M.A.N. path finding algorithm.

The prefix is better suited to mark files that are used to implement the main
part of the path finding. All other files should not use it and therefore gets
renamed.

Signed-off-by: Sven Eckelmann <sven@narfation.org>