LU-13799 llite: Modify AIO/DIO reference counting

LU-13799 llite: Modify AIO/DIO reference counting

For DIO pages, it's enough to have a reference on the
cl_object associated with the AIO.  This saves taking a
reference on the cl_object for each page, which saves about
5% of the time when doing DIO/AIO.

This is possible because the lifecycle of the aio struct is
always greater than that of the associated pages.

This patch reduces i/o time in ms/GiB by:
Write: 6 ms/GiB
Read: 1 ms/GiB

Totals:
Write: 198 ms/GiB
Read: 197 ms/GiB

mpirun -np 1  $IOR -w -r -t 64M -b 64G -o ./iorfile --posix.odirect

With previous patches in series:
write     5030 MiB/s
read      5174 MiB/s

Plus this patch:
write     5183 MiB/s
read      5200 MiB/s

Signed-off-by: Patrick Farrell <farr0186@gmail.com>
Change-Id: I970cda20417265b4b66a8eed6e74440e5d3373b8
Reviewed-on: https://review.whamcloud.com/39442
Reviewed-by: Wang Shilong <wshilong@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>