Fix unregistering memory ranges from UFFD when expanding the balloon

[maggie-lou]

Changes

When using a UFFD handler to restore a snapshot with a memory balloon, do not mmap anonymous regions over removed addresses.

Reason

We are using a UFFD handler with a memory balloon to track removed which pages were removed in the guest. When an anonymous region is mmapped over the pre-existing one, that region is deregistered from uffd and uffd will no longer receive events for that memory range. (In other words, we would expect UFFD to receive an EVENT_REMOVE for the memory ranges that the balloon has expanded into. However because the memory was remapped, we never receive events for that range.)

Steps to recreate

In this example, when using firecracker v1.10.0, you can see that the UFFD handler never receives any remove events. When using this patched version, the handler will correctly start receiving and handling remove events.

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