batman-adv: Switch to linux/array_size.h

The commit 3cd39bc3 ("kernel.h: Move ARRAY_SIZE() to a separate
header") introduced a new header for the ARRAY_SIZE macro which was
previously exposed via linux/kernel.h.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>