md: fix move_object

Recovery thread and main thread might try to recover the same object.
Either one succeeds, the other will fail and proceed and end up trying
to move the object to where it is already in place, in this case we
simply return.

Signed-off-by: Liu Yuan <tailai.ly@taobao.com>
Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>