Using direct i/o to copy file contents during migration minimizes
cache interference, but may significatly reduce performance.
Introduce new option -D/--non-direct to lfs migrate/lfs_migrate that
leaves the tradeoff at the discretion of the caller.

Signed-off-by: Daniel Kobras <d.kobras@science-computing.de>
Signed-off-by: Emoly Liu <emoly.liu@intel.com>
Change-Id: I9c2935ff204ea5385bfc38006c5476b956deb6a7
Reviewed-on: https://review.whamcloud.com/30301


Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>