mm/kmemleak: move the initialisation of object to __link_object

mm/kmemleak: move the initialisation of object to __link_object

In patch (mm: kmemleak: split __create_object into two functions), the
initialisation of object has been splited in two places.  Catalin said it
feels a bit weird and error prone.  So leave __alloc_object() to just do
the actual allocation and let __link_object() do the full initialisation.

Link: https://lkml.kernel.org/r/20231023025125.90972-1-liushixin2@huawei.com
Signed-off-by: Liu Shixin <liushixin2@huawei.com>
Suggested-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>