mm/slub: remove unneeded return value of slab_pad_check

The return value of slab_pad_check is never used. So we can make it return
void now.

Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Reviewed-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Link: https://lore.kernel.org/r/20220419120352.37825-1-linmiaohe@huawei.com