ceph: optimizing cap reservation

When caps_avail_count is in a low level, most newly
trimmed caps will probably go into ->caps_list and
caps_avail_count will be increased. Hence after trimming,
should recheck caps_avail_count to effectly reuse
newly trimmed caps. Also, when releasing unnecessary
caps follow the same rule of ceph_put_cap.

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