LU-1881 mdd: cleanup partial modification

LU-1881 mdd: cleanup partial modification

Usually, the modification triggered by client will be split into
several sub-operations in MDD layer and processed one by one. If
some step failed, we should rollback the former sub-operation(s)
to avoid partial modification the target(s) in RAM or on disk.

Signed-off-by: Fan Yong <yong.fan@whamcloud.com>
Change-Id: I02559618d252f65cd3ae3b26bd186b77c9f42dcc
Reviewed-on: http://review.whamcloud.com/3981
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Niu Yawei <niu@whamcloud.com>