LU-3534 out: only start transaction for modified update

LU-3534 out: only start transaction for modified update

Only start transaction, if the update will modify the
filesystem.

Signed-off-by: wang di <di.wang@intel.com>
Change-Id: I9c516b5fe6f6bc743e1dbc69ab04fd7062b2311d
Reviewed-on: http://review.whamcloud.com/7983
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>