batman-adv: fix alignment for batadv_coded_packet

The compiler may decide to pad the structure, and then it does not
have the expected size of 46 byte. Fix this by moving it in the
pragma pack(2) part of the code.

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