batman-adv: add build checks for packet sizes

With unrolling the batadv_header into the respective structures, the
offsetof checks are now useless. Instead, add build checks for all
packet types which go over the wire to avoid problems with wrong sizes
or compatibility issues on some architectures which don't use every day.

Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Antonio Quartulli <antonio@meshcoding.com>