batman-adv: support array of debugfs general attributes

This patch adds support for an array of debugfs general (not soft_iface
specific) attributes. With this change it will be possible to add more general
attributes by simply appending them to the array without touching the rest of
the code.

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