fix: only close `fanotify` events with a valid fd

[tombl]

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What does this PR do

fanotify returns events with the fd set to FAN_NOFD when the queue overflows. Trying to close it panics with EBADF, so don't bother.

Checklist:

• [x] I have read CONTRIBUTING.md
• [ ] I have written necessary tests and rustdoc comments
• [x] A change log has been added if this PR modifies nix's API