Make fstat work on file descriptor with no name

[hoodmane]

This also is a bit more efficient: rather than taking the stream and converting it to a path, then calling lookupPath to find the node and calling getattr on it, just lookup stream.node and call getattr on it directly.

[hoodmane]

Looks like to fix this in NODEFS, we need to make stream_ops.getattr and stream_ops.setattr and have fstat, fchmod, fchown, and ftruncate use them instead of the corresponding node_ops if they exist.