fusefs: fix intermittency in the dev_fuse_poll test

[asomers]

Summary

The DevFusePoll::access/select test would occasionally segfault. The
cause was a file descriptor that was shared between two threads. The
first thread would kill the second and close the file descriptor. But
it was possible that the second would read the file descriptor before it
shut down. That did not cause problems for kqueue, poll, or blocking
operation, but it triggered segfaults in select's macros.

MFC after: 2 weeks