LU-2056 mdd: create transaction before locking object

LU-2056 mdd: create transaction before locking object

OSD requires transactions to be created before locking the object.
however, mdd_close has a rare call path violating this rule.

Signed-off-by: Liang Zhan <liang@whamcloud.com>
Signed-off-by: Hongchao Zhang <hongchao.zhang@whamcloud.com>
Change-Id: Ibf7930338c3d1216b6ec1e0c51403ab4b8f3b940
Reviewed-on: http://review.whamcloud.com/4461
Tested-by: Hudson
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>