osd/PrimaryLogPG: fix potential pg-log overtrimming

osd/PrimaryLogPG: fix potential pg-log overtrimming

In https://github.com/ceph/ceph/pull/21580 I set a trap to catch some wired
and random segmentfaults and in a recent QA run I was able to observe it was
successfully triggered by one of the test case, see:

```
http://qa-proxy.ceph.com/teuthology/xxg-2018-07-30_05:25:06-rados-wip-hb-peers-distro-basic-smithi/2837916/teuthology.log
```

The root cause is that there might be holes on log versions, thus the
approx_size() method should (almost) always overestimate the actual number of log entries.
As a result, we might be at the risk of overtrimming log entries.

https://github.com/ceph/ceph/pull/18338 reveals a probably easier way
to fix the above problem but unfortunately it also can cause big performance regression
and hence comes this pr..

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