mm/slub: Simplify __kmem_cache_alias()

There is no need to do anything if sysfs_slab_alias() return nonzero
value after getting a mergeable cache.

Signed-off-by: Xiongwei Song <xiongwei.song@windriver.com>
Reviewed-by: Muchun Song <songmuchun@bytedance.com>
Link: https://lore.kernel.org/all/e5ebc952-af17-321f-5343-bc914d47c931@suse.cz/


Signed-off-by: Vlastimil Babka <vbabka@suse.cz>