[Virtio-fs] [virtiofsd] MR merged: passthrough: Use application-specific inode storage

Use an application-specific InodeStore struct instead of using the
more generic `MultikeyBTreeMap`.  The generic solution wasn't used
for anything except for storing inodes, and thus didn't compensate
the disadvantage of a less fitting API through the advantage of
code reuse.
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https://gitlab.com/virtio-fs/virtiofsd/-/merge_requests/88