LU-3817 llite: Truncate to restore file

LU-3817 llite: Truncate to restore file

Truncate up is safe so it won't trigger restore.

Copy optimization for truncate down - only copy the part under
truncate length. If a file is truncated to zero usually it'll be
followed by write so I choose to restore the file and set correct
stripe information.

Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Change-Id: I5e64893bdeaeb7323566154e3469f3a0ce7797d4
Reviewed-on: http://review.whamcloud.com/7505
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: jacques-Charles Lafoucriere <jacques-charles.lafoucriere@cea.fr>
Reviewed-by: Henri Doreau <henri.doreau@cea.fr>
Reviewed-by: Aurelien Degremont <aurelien.degremont@cea.fr>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>