fio: support io_size=N% (N <= 100)

fio: support io_size=N% (N <= 100)

Parse "io_size=N%".

Semantics is "multiply whatever size= calculations result in".

Example #1:

	size=50%
	io_size=50%

will do 25% of a file.

Example #2:

	size=1G
	io_size=50%

will do 512M I/O.

As side effect, fix a bug with essentially infinite loop if both size=N%
and io_size=M% are given: io_size is set to 2^64-... in this case (a lot!).

Note: only values under 100% work currently.
Going for io_size=150% requires resetting workload generator state
which is whole separate endeavour.

Signed-off-by: Alexey Dobriyan (SK hynix) <adobriyan@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>