mm, slab: use kmem_cache_free() to free from kmem_buckets_cache

mm, slab: use kmem_cache_free() to free from kmem_buckets_cache

In kmem_buckets_create(), the kmem_buckets object is allocated by
kmem_cache_alloc() from kmem_buckets_cache, but in the failure case,
it's freed by kfree(). This is not wrong, but using kmem_cache_free()
is the more common pattern, so use it.

Signed-off-by: Yan Zhen <yanzhen@vivo.com>
Reviewed-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>