ceph: refactor ceph_unreserve_caps()

The code of ceph_unreserve_caps() and error handling in
ceph_reserve_caps() are duplicated, so introduce a helper
__ceph_unreserve_caps() to reduce duplicated code.

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