batman-adv: Don't leak information through uninitialized packet fields

The reserved fields in batman-adv packets are not set to a constant value. The
content of these memory regions is leaked unintentionally to the network.

This regression was introduced in 3b27ffb0



Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Antonio Quartulli <ordex@autistici.org>