b2: fix 1TB+ uploads

Before this change the minimum chunk size would default to 96M which
would allow a maximum size of just below 1TB file to be uploaded, due to
the 10000 part rule for b2.

Now the calculated chunk size is used so the chunk size can be 5GB
making a max file size of 50TB.

Fixes #8460

backend/b2/b2.go

@@ -2224,13 +2224,17 @@ func (f *Fs) OpenChunkWriter(ctx context.Context, remote string, src fs.ObjectIn
 		return info, nil, err
 	}
 
+	up, err := f.newLargeUpload(ctx, o, nil, src, f.opt.ChunkSize, false, nil, options...)
+	if err != nil {
+		return info, nil, err
+	}
+
 	info = fs.ChunkWriterInfo{
-		ChunkSize:   int64(f.opt.ChunkSize),
+		ChunkSize:   up.chunkSize,
 		Concurrency: o.fs.opt.UploadConcurrency,
 		//LeavePartsOnError: o.fs.opt.LeavePartsOnError,
 	}
-	up, err := f.newLargeUpload(ctx, o, nil, src, f.opt.ChunkSize, false, nil, options...)
-	return info, up, err
+	return info, up, nil
 }
 
 // Remove an object