batman-adv: Calculate sizeof using variable insead of types

Documentation/CodingStyle recommends to use the form

	p = kmalloc(sizeof(*p), ...);

to calculate the size of a struct and not the version where the struct
name is spelled out to prevent bugs when the type of p changes. This
also seems appropriate for manipulation of buffers when they are
directly associated with p.

Signed-off-by: Sven Eckelmann <sven@narfation.org>