[Virtio-fs] [virtiofsd] MR merged: Add statx musl support

[virtiofs-bot at sinrega.org]

This MR adds musl support, adding the missing bit to make statx() available for the musl compilation target.

And removes the unused stat64() function. Currently, MountFds::get() uses statx() without checking its availability. But, even though PassthroughFs::new() checks if statx() is available, stat64() always returns a mount ID 0. This silently breaks '--announce-submounts', and with '--inode-file-handles' virtiofsd fails with a cryptic error, during mount.

Closes #27
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https://gitlab.com/virtio-fs/virtiofsd/-/merge_requests/99