LU-15811 llite: Rework upper/lower DIO/AIO

LU-15811 llite: Rework upper/lower DIO/AIO

One of the patches for LU-13799,
"Implement lower/upper aio"
(https://review.whamcloud.com/44209/) created a
complicated setup where the cl_dio_aio struct was used
both for the top level DIO or AIO and for the lower level
sub I/Os (corresponding to stripes).

This is quite complicated and hard to follow, so this
rewrites these two uses to be separate structs.  This
incidentally fixes at least one possible memory leak, but
is mostly a cleanup.

Fixes: 46ff761371 "LU-13799 Implement lower/upper aio"
Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: Ide4a2b84f48624ee97dfb57fe80d201fbb7fe8d0