Default buffer_compress_chunk to 512

Default buffer_compress_chunk to 512

This makes us behave better out of the box, for the cases where
people use bigger block sizes. Without this change and if the
user set buffer_compress_percentage=50 and bs=128k, then we'd
have 64k of zeroes at the end. This is probably not what the
user expects.

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