staging: lustre: 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>
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-3817
Reviewed-on: http://review.whamcloud.com/7505


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>
Signed-off-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>