New offset_increment option

New offset_increment option

This patch adds a new option to fio job files. It is described
in the HOWTO as follows:

offset_increment=int	If this is provided, then the real offset becomes
		the offset + offset_increment * thread_number, where the
		thread number is a counter that starts at 0 and is incremented
		for each job. This option is useful if there are several jobs
		which are intended to operate on a file in parallel in disjoint
		segments, with even spacing between the starting points.

Signed-off-by: Jens Axboe <axboe@kernel.dk>