Avoid extra check for clean object

We needn't to check clean object via buffer state, skip the clean object.

Signed-off-by: Haomai Wang <haomaiwang@gmail.com>
Merge pull request #1848 from yuyuyu101/librbd-flush

Avoid extra check for clean object

Backport: firefly, dumpling
Reviewed-by: Sage Weil <sage@inktank.com>