LU-2700 mdt: serializing the rename

LU-2700 mdt: serializing the rename

BFL is needed in the rename process even though the source and
target are under the same directory, because we still need
serialize the locking of these two objects.

Signed-off-by: wang di <di.wang@intel.com>
Change-Id: I61593e3902f1f81ac282c020d912fdf28ce2cc2c
Reviewed-on: http://review.whamcloud.com/5203
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Tested-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>