cinder gets out of memory at json.dumps when doing a backup of a large volume

[David Caro]

We are running cinder 17.2.0-1 on debian, using posix backend, and when trying to do a backup of a huge volume we always hit the error:
```
                                                      2022-03-05 08:36:23.108 18548 ERROR oslo_messaging.rpc.server File "/usr/lib/python3/dist-packages/cinder/backup/chunkeddriver.py", line 289, in _write_sha256file
                                                      2022-03-05 08:36:23.108 18548 ERROR oslo_messaging.rpc.server sha256file_json = json.dumps(sha256file, sort_keys=True, indent=2)
                                                      2022-03-05 08:36:23.108 18548 ERROR oslo_messaging.rpc.server File "/usr/lib/python3.7/json/__init__.py", line 238, in dumps
                                                      2022-03-05 08:36:23.108 18548 ERROR oslo_messaging.rpc.server **kw).encode(obj)
                                                      2022-03-05 08:36:23.108 18548 ERROR oslo_messaging.rpc.server File "/usr/lib/python3.7/json/encoder.py", line 202, in encode
                                                      2022-03-05 08:36:23.108 18548 ERROR oslo_messaging.rpc.server return ''.join(chunks)
                                                      2022-03-05 08:36:23.108 18548 ERROR oslo_messaging.rpc.server MemoryE
```

As there's a spike of memory usage right there (join of the chunks duplicates it).

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/c/openstack/cinder/+/834527