LU-4569 hsm: Prevent copytool from importing existing file.

LU-4569 hsm: Prevent copytool from importing existing file.

Requesting import of an already existing FID can lead to data
corruption since both inodes in lustre would refer to the same
file in the backend.

Signed-off-by: Henri Doreau <henri.doreau@cea.fr>
Change-Id: I38bab1c890ff84b22f8816597f6d0b367aba4b38
Reviewed-on: http://review.whamcloud.com/10185
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: jacques-Charles Lafoucriere <jacques-charles.lafoucriere@cea.fr>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>