fusefs: multiple fixes related to the write cache
- Don't always write the last page synchronously. That's not actually required. It was probably just masking another bug that I fixed later, possibly in r349021.
- Enable the NotifyWriteback tests now that Writeback cache is working.
- Add a test to ensure that the write cache isn't flushed synchronously when in writeback mode.
Sponsored by: The FreeBSD Foundation