os/Newstore: flush_commit return true on STATE_KV_DONE

os/Newstore: flush_commit return true on STATE_KV_DONE

There is a racing condition here, if the flush_commit() call
happened after _txc_finish_kv and before next state, the context
was pushed to on_commits but no one will handle the context since
we already pass _txc_finish_kv. This bug can be easily reproduce
by putting a sleep(5) after _txc_finish_kv, and trigger the bug by
ceph-osd -i 0 --mkfs.

Fix this bug by return true directly when state >= STATE_KV_DONE(instead
of > in previous code). We already persist the data in STATE_KV_DONE so
it's safe for us to do this.

Signed-off-by: Xiaoxi Chen <xiaoxi.chen@intel.com>