Fix default scrambling of buffers

Commit 1bf2498d3111 inadvertently made us not scramble by default.
The logic was a bit convoluted, so clear it up and scramble if:

- Asked so by adding scramble_buffers=1
- Or, if not asked to explicitly, but zero_buffers not set to 1.

Reported-by: Chris Wilson <Chris.Wilson@nutanix.com>
Signed-off-by: Jens Axboe <axboe@fb.com>