io_u: if we're doing backwards IO, wrap to end (not start)

io_u: if we're doing backwards IO, wrap to end (not start)

If we have a job with a start file offset and we're doing holed
IO, we want to wrap to the start offset for forwards holes, but
to the end for backwards holes.

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