LU-13814 osc: Remove most uses of oap_obj

LU-13814 osc: Remove most uses of oap_obj

Removing most uses of oap_obj makes it easier to do the
upcoming transient page cl_page removal.

Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: Ic8acaed2ce3c6831f9a0d2bd13d859b9c564efdd
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/52072
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>