Use PreallocatedOverlapped when internal FileStream buffer isn't being used on Windows

[davidfowl]

See comment here dotnet/coreclr#16190 (comment)

/cc @stephentoub

[stephentoub]

Thanks, @davidfowl.

Am I correct in remembering that the FileStreams ASP.NET cares most about for performance are created with a buffer size of 1 in order to avoid having an internal buffer? If so, I think we should do (3) from my linked comment, simply special-casing buffer size of 1 as the common way of avoiding the buffer, and just changing FileStream's GetBuffer to use Array.Empty and initialize the preallocated. Then we can again allow a null array coming from Memory usage to use the preallocated overlapped.

cc: @JeremyKuhne