Fix rbd bench-write improper behavior

"rbd bench-write" eject all write operations with the same offset at the same
time. It will result in non-objective performance result from this command.

fix #7066

Co-Author: Rongze Zhu <rongze@unitedstack.com>
Signed-off-by: Haomai Wang <haomaiwang@gmail.com>
Merge pull request #1006 from yuyuyu101/wip-7066

Fix rbd bench-write improper behavior

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>