os/bluestore: add compression required ratio to enable/disable compression

os/bluestore: add compression required ratio to enable/disable compression

Require the net gain of compression at least to be at a specified ratio,
otherwise we don't compress.

Ask for compressing at least 12.5% off, by default.

This is for the sake of performance because if the compression turns out
to be meaningless(saving little space), we can simply shut it down, as we
know the compression/decompression can be rather CPU-consuming.

Signed-off-by: xie xingguo <xie.xingguo@zte.com.cn>