SMB: Creating named streams on existing files is not quite right
In MS-FSA, 'file lookup' effectively has the following branches:
1. If the file doesn't exist, create the file and the stream
2. If the file exists, and the stream exists, perform access and sharing checks
3. If the file exists, but the stream doesn't exist, perform access checks, then create the stream.
Currently, 1 and 3 are collapsed into a single branch: If the file exists, but the stream doesn't, that's treated as if the file didn't exist.