[seaweedfs] Fix setting size for multi-dc volumes (#1476)

Signed-off-by: Andrei Kvapil <kvapss@gmail.com>

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## What this PR does


### Release note

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```release-note
[seaweedfs] Fix setting size for multi-dc volumes
```

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-->
## Summary by CodeRabbit

* **Bug Fixes**
* Per-zone data directory size now falls back to the global volume size
value when a zone doesn’t specify one, fixing incorrect fallback
behavior.
* Users relying on the previous fallback may need to set zone-specific
sizes to preserve prior behavior.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/extra/seaweedfs/templates/seaweedfs.yaml

@@ -106,7 +106,7 @@ spec:
             {{- if $zone.size }}
             size: "{{ $zone.size }}"
             {{- else }}
-            size: "{{ $.Values.size }}"
+            size: "{{ $.Values.volume.size }}"
             {{- end }}
             {{- if $zone.storageClass }}
             storageClass: {{ $zone.storageClass }}