[Virtio-fs] [virtiofsd] MR opened: passthrough: Reset config on FUSE_DESTROY

[virtiofs-bot at sinrega.org]

`PassthroughFs::{writeback, announce_submounts}` are set (as the
circumstances require) in `init()`.  Therefore, we also reset them
in `destroy()`.  Without this commit, those variables would always
be set after they're set once.  This might be undesired when the
caller calls `destroy()` and then `init()` with deviating
`FsOptions`.
---
https://gitlab.com/virtio-fs/virtiofsd/-/merge_requests/84