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