rbd_obj_request_wait() should cancel the underlying OSD request if
interrupted.  Otherwise libceph will hold onto it indefinitely, causing
assert failures or leaking the original object request.

This also adds an rbd wrapper around ceph_osdc_cancel_request() to
match rbd_obj_request_submit() and rbd_obj_request_wait().
Signed-off-by: NIlya Dryomov <ilya.dryomov@inktank.com>
Reviewed-by: NAlex Elder <elder@linaro.org>