Fix FileStream.FlushAsync() to behave like Flush() (dotnet/coreclr#24902)

Fix FileStream.FlushAsync() to behave like Flush() (dotnet/coreclr#24902)

Flush() behaves like Flush(false) and writes out any buffered data but doesn't P/Invoke to FlushFileBuffers/FSync to flush the OS buffers.

But whereas FlushAsync() is supposed to just be an async equivalent of Flush(), it's actually behaving like Flush(true).  This makes FlushAsync() inconsistent and much more expensive.  (This is separate from FlushAsync not actually being async, which is an impactful problem to be solved separately.)

This changes FlushAsync to behave like Flush()/Flush(false) rather than Flush(true).  If someone wants the FlushFileBuffers/FSync behavior, they can call Flush(true).

Commit migrated from https://github.com/dotnet/coreclr/commit/436debb93c390e144f9c25b5d1922ac7780745f4