[leo_gateway][nfs] NFS take disk space a little bit more than S3/REST

[mocchira]

When creating a new file via NFS, NFS client always issues create and at this point leofs create a new empty file so that the size of AVS header take space and subsequent writes are written along with a new AVS header. That's why NFS take disk space a little bit (the size of AVS header per file) more than others. so the active disk usage may get worse in case the average file size is quite small.

I think it's just spec but we need to document.