Scramble default write buffer contents slightly

If we are worried about de-dupe making the output data too easy
to compress, but don't want the full incompressible guarantee
provided by refill_buffers=1 (since it's too costly), then lets
make their life a little harder by filling the offset of the IO
into the beginning of the buffer and the start time at the end
of the buffer. This will effectively default simple de-dupe
or compression attempts, while not doing the costly full buffer
refill.

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