LU-13799 clio: Skip prep for transients

LU-13799 clio: Skip prep for transients

The work done by cpo_prep() (etc) is unnecessary for
transient pages.  This gives only a minimal performance
boost and is better seen as a step towards removing the
cl_page abstraction for transient pages.

But, it does consistently give around 1% better
performance.

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

Totals:
Write: 169 ms/GiB
Read: 161 ms/GiB

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

With previous patches in series:
write        6028 MiB/s
read         6305 MiB/s

Plus this patch:
write        6071 MiB/s
read         6355 MiB/s

Signed-off-by: Patrick Farrell <farr0186@gmail.com>
Change-Id: Ib94f57cde468c9aaea952e1bb89db8fcf4b35e07
Reviewed-on: https://review.whamcloud.com/39448
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>