io_u: ensure that we align new start offset properly for time_based

io_u: ensure that we align new start offset properly for time_based

If io_size isn't a multiple of the block size, weird things can
happen, which usually means that fio will exit with an unaligned
IO error:

fio: io_u error on file /dev/nvme0n1: Invalid argument: write offset=125830, buflen=1048576

where offset=125830 really should have been aligned to the block
size. Ensure that we do that.

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