Update VFS interface to optionally return object attributes

[kofemann]

In many cases, after creating a new file or directory, the nfs layer queries for file attributes of the newly created object. Typically, the filesysem implementation alredy has those attributes, however create operations return only the inode, thus extra getattr is required.

I propose to change the interface and instead of an Inode retun a CreateResult, like:

record CreateResult (Inode inode, Optional<Stat> stat) {} ;

If filesysem cant provide stat, then the will be empty, and calling code will fall back calling getattr.