LU-13134 obdclass: use slab allocation for cl_dio_aio

cl_dio_aio is used frequently for dio/aio, try to use
a private slab pool for it.

This could help improve aio performance.

Change-Id: Ic06523ae59eed04e55c17ac03af9187af8f695c5
Signed-off-by: Wang Shilong <wshilong@ddn.com>
Reviewed-on: https://review.whamcloud.com/37227
Reviewed-by: Patrick Farrell <farr0186@gmail.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>