ceph: remove request from waiting list before unregister

Link: https://tracker.ceph.com/issues/40339


Signed-off-by: "Yan, Zheng" <zyan@redhat.com>
Reviewed-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>