LU-2883 hsm: Mark file DIRTY as soon as pages are written

LU-2883 hsm: Mark file DIRTY as soon as pages are written

Since the dirty flag has to be packed in close, it should be set when
pages are written and not when building BRWs like SOM which relies on
MDS_DONE_WRITING.

Signed-off-by: Aurelien Degremont <aurelien.degremont@cea.fr>
Change-Id: I9cf0a71cf3228a7aadb8205cff2735a7abff5ef0
Reviewed-on: http://review.whamcloud.com/5543
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: Johann Lombardi <johann.lombardi@intel.com>