libceph: fix misleading ceph_osdc_cancel_request() comment

cancel_request() never guaranteed that after its return the OSD
client would be completely done with the OSD request.  The callback
(if specified) can still be invoked and a ref can still be held.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Xiubo Li <xiubli@redhat.com>