fusefs: Fix intermittency in the BadServer.ShortWrite test case

[asomers]

Summary

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 <[e-mail]>
MFC after: 1 week

[guest-svmhdvn]

Seems to fix the intermittency even with a very long runtime. I'm not the best person to review the changes, but it works well for me.