slub: Change get_partial() interfaces to return slab

We need all get_partial() related interfaces to return a slab, instead
of returning the freelist (or object).

Use the partial_context.object to return back freelist or object for
now. This patch shouldn't have any functional changes.

Suggested-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Tested-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Reviewed-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>