osd/ReplicatedPG: fix objecter locking in start_flush

Broken in backport fd96eb62ece27f5c660429584c2ff2e058bc6e94.

Signed-off-by: Sage Weil <sage@redhat.com>
Merge pull request #2576 from ceph/wip-9593

osd/ReplicatedPG: fix objecter locking in start_flush

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>