LU-13814 clio: cleanup cl_page_completion

LU-13814 clio: cleanup cl_page_completion

Clean up cl_page_completion and make very explicit which
parts of the function do not apply to transient pages.

Signed-off-by: Patrick Farrell <patrick.farrell@oracle.com>
Change-Id: Ic288717c8487ff963f0fa7f63a943e72d05d129a
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/52084
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Qian Yingjin <qian@ddn.com>
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>