filesetup: add fallocate=truncate option.

filesetup: add fallocate=truncate option.

Fixes #833. Provides the ability to initially "layout" the file
by ftruncating it to the desired size before performing IO. This
is mainly useful on Windows, which serializes all writes that
extend the size of a file. Using this option with a suitable
iodepth allows fio to emulate the behavior of Windows Explorer
file copy, which always truncates to the expected size before
issuing writes for performance reasons.