batman-adv: Use packing of 2 for all headers before an ethernet header

All packet headers in front of an ethernet header have to be completely
divisible by 2 but not by 4 to make the payload after the ethernet header again
4 bytes boundary aligned.

A packing of 2 is necessary to avoid extra padding at the end of the struct
caused by a structure member which is larger than two bytes. Otherwise the
structure would not fulfill the previously mentioned rule to avoid the
misalignment of the payload after the ethernet header. It may also lead to
leakage of information when the padding it not initialized before sending.

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