io_u: tweak small content buffer scramble

io_u: tweak small content buffer scramble

We currently generate a 'random' offset in a 512b chunk to fill in
the offset. Since we don't want the later time scramble to
overwrite it, we check and adjust for that. Instead just ensure that
we generate a random offset in the first half of the 512b chunk,
then we know we never overlap.

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