LU-13134 llite: fix to free cl_dio_aio properly

LU-13134 llite: fix to free cl_dio_aio properly

@cl_dio_aio is allocated by slab, we should use slab
free helper to free its memory.

Fixes: 3db1d ("LU-13134 obdclass: use slab allocation for cl_dio_aio")
Signed-off-by: Wang Shilong <wshilong@ddn.com>
Change-Id: Ic82f4dec5e9bb6de7a531c224fa23248d4f7d63d
Reviewed-on: https://review.whamcloud.com/39103
Reviewed-by: Patrick Farrell <farr0186@gmail.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>