osd: fix unordered read bug (for chunked object)

osd: fix unordered read bug (for chunked object)

The current implementation for chunked object only supports
proxy_read(if offset is within range) and write(local write)
In this case, a read request can be handled before a write request.
This commit prevents unordered read processing because
proxy_read() will be executed if the chunk is missing state.
If chunked object has been overwritten, its state will not be missing.

Fixes: http://tracker.ceph.com/issues/22369
Signed-off-by: Myoungwon Oh <omwmw@sk.com>