LU-13799 lov: Improve DIO submit

LU-13799 lov: Improve DIO submit

Skip some unnecessary looping in page submission for the
DIO case.

This gives about a 2% improvement for AIO/DIO page
submission.

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

Totals:
Write: 172 ms/GiB
Read: 165 ms/GiB

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

With previous patches in series:
write        7726 MiB/s
read         5899 MiB/s

Plus this patch:
write        5954 MiB/s
read         6217 MiB/s

Signed-off-by: Patrick Farrell <farr0186@gmail.com>
Change-Id: Iedad978438ee3f1f3290d990311532626cba9e2d
Reviewed-on: https://review.whamcloud.com/39446
Reviewed-by: Wang Shilong <wshilong@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>