LU-220 open: try open_by_fid for the known object

LU-220 open: try open_by_fid for the known object

If the current path for thread1 on client1 is renamed by thread2
on client2, then when thread1 tries to open its current path, it
should use the fid, but not the name. Otherwise, the open on MDS
will hit unexpected "-ENOENT".

Signed-off-by: Fan Yong <yong.fan@whamcloud.com>
Change-Id: I905113167ebd6fe5f0185007d0e9c77083342680
Reviewed-on: http://review.whamcloud.com/2493
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>