LU-3718 mdt: HSM EXIST event not triggered at last rm/mv

LU-3718 mdt: HSM EXIST event not triggered at last rm/mv

After a rm or mv, if the target file is destroyed
(last unlink) and if there is an archive in the backend
a flags must be set in changelog event

Signed-off-by: JC Lafoucriere <jacques-charles.lafoucriere@cea.fr>
Change-Id: I92f3ea91d377ba8b3724aef43d4e7ba8b0a6f681
Reviewed-on: http://review.whamcloud.com/7260
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>