In the license file that comes with the WDK, section 2a I, second bullet point.

You may modify, copy, and distribute the source and object code form of code listed in the SAMPLES.TXT file.  You may modify, copy, and distribute only the object code form of other sample code that is not listed in the SAMPLES.TXT file.

The fastfat driver is not listed in the SAMPLES.TXT file, so that seems to imply we cannot redistribute the source.  Unless you're suggesting we don't care about not being able to distribute code we include with ReactOS?