Allow metadata updates on meta bucket even in WORM mode (#8657)

This ensures that we can update the

- .minio.sys is updated for accounting/data usage purposes
- .minio.sys is updated to indicate if backend is encrypted
  or not.

cmd/xl-v1-multipart.go

@@ -708,7 +708,7 @@ func (xl xlObjects) CompleteMultipartUpload(ctx context.Context, bucket string,
 
 	if xl.isObject(bucket, object) {
 		// Deny if WORM is enabled
-		if globalWORMEnabled {
+		if _, ok := isWORMEnabled(bucket); ok {
 			if _, err := xl.getObjectInfo(ctx, bucket, object); err == nil {
 				return ObjectInfo{}, ObjectAlreadyExists{Bucket: bucket, Object: object}
 			}