LU-15811 llite: Refactor DIO/AIO free code

LU-15811 llite: Refactor DIO/AIO free code

Refactor the DIO/AIO free code and add some asserts.

This removes a potential use-after-free in the freeing
code.

Lustre-change: https://review.whamcloud.com/48115/
Lustre-commit: 0358bd41174176cbfc9d6786bffb6dc95b68adcf (tbd)

Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I335b18fc7a28fc426a25675e2449d3d192cba596
Reviewed-on: https://review.whamcloud.com/48103
Reviewed-by: Yingjin Qian <qian@ddn.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Andreas Dilger <adilger@whamcloud.com>