mm/slub: remove unused parameter in next_freelist_entry()

The parameter "struct slab *slab" is unused in next_freelist_entry(),
so just remove it.

Acked-by: Christoph Lameter (Ampere) <cl@linux.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>