aio: remove the needless registration of ring file's private_data

Remove the registration of ring file's private_data, we do not use
it.
Reviewed-by: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Gu Zheng <guz.fnst@cn.fujitsu.com>
Signed-off-by: Benjamin LaHaise <bcrl@kvack.org>

fs/aio.c

@@ -192,7 +192,6 @@ static struct file *aio_private_file(struct kioctx *ctx, loff_t nr_pages)
 	}
 
 	file->f_flags = O_RDWR;
-	file->private_data = ctx;
 	return file;
 }