mm, slab: fix bad alignments

As reported by coccicheck:

./mm/slab.c:3253:2-59: code aligned with following code on line 3255.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Acked-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Acked-by: David Rientjes <rientjes@google.com>
Reviewed-by: Muchun Song <songmuchun@bytedance.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>