LU-13814 clio: rename 'cl_page_completion'

LU-13814 clio: rename 'cl_page_completion'

All of the other cl_page operations are verbs, so let's
make this one a verb as well rather than the slightly odd
'completion'.

Test-Parameters: trivial
Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I3f52372b4e9ae1805fdfddb31c908553a381c0aa
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/52112
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Qian Yingjin <qian@ddn.com>
Reviewed-by: Marc Vef <mvef@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>