To properly clear the slab on free with slab_want_init_on_free, we walk
the list of free objects using get_freepointer/set_freepointer.
The value we get from get_freepointer may not be valid. This isn't an
issue since an actual value will get written later but this means
there's a chance of triggering a bug if we use this value with
set_freepointer: