File Object Panics on Disconnect

Reproducing the bug

Start NBD Server with Raw Block + File Object inside a folder:

nbd-rs --export myexport raw file:///folder/file

Connect to the NBD Server with the export myexport

nbd-client -N myexport localhost /dev/nbd0

Disconnect from the NBD Server

nbd-client -d /dev/nbd0

Results:

thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Os { code: 2, kind: NotFound, message: "No such file or directory" }', src/object/file.rs:213:55
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

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