Don't decrease FileSize on write

[saibotu]

This prevents decreasing the reported FileSize/EndOfFile when only part of a file is written, which fixes compatibility with various applications.

[billziss-gh]

@saibotu thanks for the PR. It does seem that you have identified a straight bug and provided a fix for it. THANK YOU!

I am not sure how this has not been caught by any tests. FSX is pretty good at catching things like this (but I note now that it is not being run for any of the example FUSE file systems). I am going to test locally and confirm the problem and its resolution.

Lastly in order for me to accept contributions to this project I require that you add your name and email address to the Contributors document: https://github.com/billziss-gh/winfsp/blob/master/Contributors.asciidoc

This effectively signs the document, assigns joint copyright to your change and identifies you as a contributor. You only need to do this one time on your first contribution.

[billziss-gh]

@saibotu thank you for signing the agreement.

I have tested the PR locally with FSX. A WinFsp-FUSE file system (e.g. passthrough-fuse) fails fast under FSX without your fix. FSX has no complaints when your fix is added.

I am accepting this PR. I have also modified the WinFsp test suite to run FSX over passthrough-fuse; this will be the next commit.