batman-adv: Fix includes for *_MAX constants

The commit 54d50897 ("linux/kernel.h: split *_MAX and *_MIN macros into
<linux/limits.h>") moved the U32_MAX/INT_MAX/ULONG_MAX from linux/kernel.h
to linux/limits.h. Adjust the includes accordingly.

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