mm, slab: Replace 'caller' type, void* -> unsigned long

This allows to use _RET_IP_ instead of builtin_address(0), thus
achiveing implementation consistency in all three allocators.
Though maybe a nitpick, the real goal behind this patch is
to be able to obtain common code between SLAB and SLUB.

Signed-off-by: Ezequiel Garcia <elezegarcia@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>