LU-3712 lmv: choose right MDT for open by FID

If the open is by FID, LMV should locate MDT by the FID
of the file/directory, instead of its parent.

Signed-off-by: wang di <di.wang@intel.com>
Change-Id: If7815de8cab0cf28b521decccda3c5f0f1aed73c
Reviewed-on: http://review.whamcloud.com/7253
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>