Compression fixes

Compression fixes

- memset() remainder of buffer instead of copying random segment.
  Provides more reliable compression (yes yes, you told me so).

- Allow 100 as well, just means full memset. Make that just set
  zero_buffers=1 instead, since we only need to do it once
  instead of for every write.

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