slub: Use freelist instead of "object" in __slab_alloc

The variable "object" really refers to a list of objects that we
are handling.

Change-Id: I4093c483a85360836bf299caa5eab40b57b02718
Signed-off-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>