LU-6142 obdclass: make ccc_users in cl_client_cache a refcount_t

LU-6142 obdclass: make ccc_users in cl_client_cache a refcount_t

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

Change-Id: I5af513ccb2b706a398e647ce0427affa4516a9b5
Signed-off-by: Mr. NeilBrown <neilb@suse.de>
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/48881
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: Emoly Liu <emoly@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>