s3: fix multitenant multipart uploads with CEPH

CEPH uses a special bucket form `tenant:bucket` for multitentant
access using S3 as documented here:

https://docs.ceph.com/en/reef/radosgw/multitenancy/#s3

However when doing multipart uploads, in the reply from
`CreateMultipart` the `tenant:` was missing from the `Bucket` response
rclone was using to build the `UploadPart` request. This caused a 404
failure return. This may be a CEPH bug, but it is easy to work around.

This changes the code to use the `Bucket` and `Key` that we used in
`CreateMultipart` in `UploadPart` rather than the one returned from
`CreateMultipart` which fixes the problem.

See: https://forum.rclone.org/t/rclone-zcat-does-not-work-with-a-multitenant-ceph-backend/48618

backend/s3/s3.go

@@ -6070,8 +6070,8 @@ func (f *Fs) OpenChunkWriter(ctx context.Context, remote string, src fs.ObjectIn
 		chunkSize:            int64(chunkSize),
 		size:                 size,
 		f:                    f,
-		bucket:               mOut.Bucket,
-		key:                  mOut.Key,
+		bucket:               ui.req.Bucket,
+		key:                  ui.req.Key,
 		uploadID:             mOut.UploadId,
 		multiPartUploadInput: &mReq,
 		completedParts:       make([]types.CompletedPart, 0),