rsync re-copies forever when target is blobfuse mount

`gsutil -m rsync -d -r gs://export /mnt/blobfuse` running forever is what caused us to notice this.

`gsutil cp` to copy the same file twice correctly returns "Download already complete for /mnt/blobfuse/file, skipping download but will run integrity checks" while rsync appears to re-copy forever.

Perhaps a filesystem call is unimplemented or partially implemented on this fuse filesystem, which should be easy to check prior to starting the rsync. `ls` gives correct results, but `du` returns zero size for blobfuse mounts. Tested on ubuntu 17.10, blobfuse 1.0.0-RC-preview, gsutil 4.30.