batman-adv: Migrate to linux/container_of.h

The commit d2a8ebbf ("kernel.h: split out container_of() and
typeof_member() macros")  introduced a new header for the container_of
related macros from (previously) linux/kernel.h.

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