LU-6142 clio: make cp_ref in cl_page a refcount_t

LU-6142 clio: make cp_ref in cl_page a refcount_t

As this is used as a refcount, it should be declared
as one.

Change-Id: I8108e14e545bb56aae34a0f6ae9d5a04227fc067
Signed-off-by: Mr. NeilBrown <neilb@suse.de>
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/49072
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>