Squashed commit of the following:
fs/fat: Resolves issues with truncating the cluster chain when shrinking files via ftruncate().
fs/fat: First cut at implementation file shrinkage logic needed to support ftruncate(). Certainly shrinks the file size but it does not appear to correctly disconnect the cluster chains.
fs/fat: Restructure some functions in files to better support forthcoming file shrinkage logic. Put framework for file shrinkage in place. That logic is incomplete on initial commit.
fb73006a4b