fusefs: Fix further intermittency in the BadServer.ShortWrite test case

After being unmounted, the mockfs server would occasionally read from
/dev/fuse again, if the main function didn't exit fast enough, getting
an ENODEV error. Handle that appropriately.

Reported by: Siva Mahadevan <me@svmhdvn.name>
Fixes: d86025c1d49c84c4dc8c3635c83c078ad56e5a53
MFC after: 1 week
Reviewed by: Siva Mahadevan <me@svmhdvn.name>
Differential Revision: https://reviews.freebsd.org/D54331