percpu-refcount, aio: use percpu_ref_cancel_init() in ioctx_alloc()

ioctx_alloc() reaches inside percpu_ref and directly frees
->pcpu_count in its failure path, which is quite gross.  percpu_ref
has been providing a proper interface to do this,
percpu_ref_cancel_init(), for quite some time now.  Let's use that
instead.

This patch doesn't introduce any behavior changes.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Benjamin LaHaise <bcrl@kvack.org>
Cc: Kent Overstreet <kmo@daterainc.com>

fs/aio.c

@@ -715,8 +715,8 @@ err_ctx:
 err:
 	mutex_unlock(&ctx->ring_lock);
 	free_percpu(ctx->cpu);
-	free_percpu(ctx->reqs.pcpu_count);
-	free_percpu(ctx->users.pcpu_count);
+	percpu_ref_cancel_init(&ctx->reqs);
+	percpu_ref_cancel_init(&ctx->users);
 	kmem_cache_free(kioctx_cachep, ctx);
 	pr_debug("error allocating ioctx %d\n", err);
 	return ERR_PTR(err);