aio: use iocb_put() instead of open coding it

commit 71ebc6fef0f53459f37fb39e1466792232fa52ee upstream.

Replace the percpu_ref_put() + kmem_cache_free() with a call to
iocb_put() instead.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Cc: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>