ceph: release unreserved caps if having enough available caps

When unreserving caps check if there is too mamy available caps
in the ->caps_list, if so release unreserved caps.

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