Devices object and DosDevices object linkage

[David J. Craig]

For any one who cares:

This has been a couple of interesting days. I needed to determine to which
drive letter our file system filter is connected. We know the device name
we filtered, but don't know the drive letter. I have determined that during
our initialization of our filter device, the object manager has not yet
assigned a drive letter. It appears that it only occurs after the device
object is created and connected to the appropriate file system.

I have found a method to do it later via ZwOpenSymbolicLink() &
ZwQuerySymbolicLink(), but it is fairly complex in that you have to do each
drive letter until the connected device is the one you were looking to find.
It isn't a lot of code, but it was a bit painful to get it working,
especially in the context of the existing code.