fusefs: Fix intermittency in the BadServer.ShortWrite test case

We were using the m_quit bit for two similar but distinct uses:

• To instruct the server to quit
• To cope with the kernel forcibly unmounting the fs

Fix the intermittent test failure by adding a separate bit,
m_expect_unmount, to handle cases like the latter.

Reported by: Siva Mahadevan <me@svmhdvn.name>
MFC after: 1 week
Revied by: Siva Mahadevan <me@svmhdvn.name>
Differential Revision: https://reviews.freebsd.org/D53357