Add option for io_limit

Add option for io_limit

'size' denotes both the size of the region for IO, as well as the
amount of IO to transfer. Sometimes it's useful to be able to set
them separately instead. With this patch, you can do:

size=100G
io_limit=10G

and have fio do 10G of IO between 0..100G of the address space.

Signed-off-by: Jens Axboe <axboe@fb.com>