libceph: fix truncate size calculation

commit ccca4e37b1a912da3db68aee826557ea66145273 upstream.

check the "not truncated yet" case

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
Reviewed-by: Sage Weil <sage@inktank.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>