LU-13799 llite: Adjust dio refcounting

LU-13799 llite: Adjust dio refcounting

We get a page reference in cl_page_find, then immediately
add another for cl_2queue_add and remove the first
reference.  This is pretty silly, since the life cycle is
the same on these.

This improves DIO/AIO page submission by around 2%.

This patch reduces i/o time in ms/GiB by:
Write: 2 ms/GiB
Read: 2 ms/GiB

Totals:
Write: 170 ms/GiB
Read: 162 ms/GiB

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

With previous pa5ches in series:
write        5955 MiB/s
read         6218 MiB/s

Plus this patch:
write        6028 MiB/s
read         6305 MiB/s

Lustre-change: https://review.whamcloud.com/39447
Lustre-commit: 1e4d10af3909452b0eee1f99010d80aeb01d42a7

Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I228eca6d48c6007bbf2c8caae5e477b7d40521d1
Reviewed-on: https://review.whamcloud.com/44446
Reviewed-by: Li Xi <lixi@ddn.com>
Tested-by: Li Xi <lixi@ddn.com>