batman-adv: fix size of batadv_bla_claim_dst

Since this is a mac address and always 48 bit, and we can assume that
it is always aligned to 2-byte boundaries, add a pack(2) pragma.

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