ceph: mark the cap cache as unreclaimable

Releasing cap is affected by many factors (e.g., avail_count/reserve_count/min_count)
and min_count could be specified high volume in client mount option. Hence it's better
to mark cap cache as unreclaimable in case of non-trivial discrepancies between memory
shown as reclaimable and what is actually reclaimed.

Signed-off-by: Chengguang Xu <cgxu519@icloud.com>
Reviewed-by: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>