Kernel: Start implementing purgeable memory support

It's now possible to get purgeable memory by using mmap(MAP_PURGEABLE).
Purgeable memory has a "volatile" flag that can be set using madvise():

- madvise(..., MADV_SET_VOLATILE)
- madvise(..., MADV_SET_NONVOLATILE)

When in the "volatile" state, the kernel may take away the underlying
physical memory pages at any time, without notifying the owner.
This gives you a guilt discount when caching very large things. :^)

Setting a purgeable region to non-volatile will return whether or not
the memory has been taken away by the kernel while being volatile.
Basically, if madvise(..., MADV_SET_NONVOLATILE) returns 1, that means
the memory was purged while volatile, and whatever was in that piece
of memory needs to be reconstructed before use.

Kernel/VM/PurgeableVMObject.h

@@ -0,0 +1,32 @@
+#pragma once
+
+#include <Kernel/VM/AnonymousVMObject.h>
+
+class PurgeableVMObject final : public AnonymousVMObject {
+public:
+    virtual ~PurgeableVMObject() override;
+
+    static NonnullRefPtr<PurgeableVMObject> create_with_size(size_t);
+    virtual NonnullRefPtr<VMObject> clone() override;
+
+    int purge();
+
+    bool was_purged() const { return m_was_purged; }
+    void set_was_purged(bool b) { m_was_purged = b; }
+
+    bool is_volatile() const { return m_volatile; }
+    void set_volatile(bool b) { m_volatile = b; }
+
+private:
+    explicit PurgeableVMObject(size_t);
+    explicit PurgeableVMObject(const PurgeableVMObject&);
+
+    PurgeableVMObject& operator=(const PurgeableVMObject&) = delete;
+    PurgeableVMObject& operator=(PurgeableVMObject&&) = delete;
+    PurgeableVMObject(PurgeableVMObject&&) = delete;
+
+    virtual bool is_purgeable() const override { return true; }
+
+    bool m_was_purged { false };
+    bool m_volatile { false };
+};