LU-13799 llite: Do not get/put DIO pages

LU-13799 llite: Do not get/put DIO pages

We've already told the kernel we're working with these pages
using the get/put_user_pages functions, and userspace must
hold references on them throughout the i/o anyway.

So getting/putting these vmpages is unnecessary.  This
saves around 7% of the time in DIO page submission, netting
about that much of a performance improvement.

This patch reduces i/o time in ms/GiB by:
Write: 22 ms/GiB
Read: 19 ms/GiB

Totals:
Write: 135 ms/GiB
Read: 143 ms/GiB

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

With previous patches in series:
write     6470 MiB/s
read      6354 MiB/s

Plus this patch:
write     7531 MiB/s
read      7179 MiB/s

Lustre-change: https://review.whamcloud.com/39438
Lustre-commit: 881b4c722296ff7ac22c6fd7988363f2cdad9f1e

Signed-off-by: Patrick Farrel <pfarrell@whamcloud.com>
Change-Id: Icfd5bc73ba4254898d6051f11ab6aea624948763
Reviewed-on: https://review.whamcloud.com/44686
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>