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/VMObject.h

@@ -22,6 +22,7 @@ public:
     virtual NonnullRefPtr<VMObject> clone() = 0;
 
     virtual bool is_anonymous() const { return false; }
+    virtual bool is_purgeable() const { return false; }
     virtual bool is_inode() const { return false; }
 
     size_t page_count() const { return m_physical_pages.size(); }
@@ -42,11 +43,10 @@ protected:
     void for_each_region(Callback);
 
     FixedArray<RefPtr<PhysicalPage>> m_physical_pages;
+    Lock m_paging_lock { "VMObject" };
 
 private:
     VMObject& operator=(const VMObject&) = delete;
     VMObject& operator=(VMObject&&) = delete;
     VMObject(VMObject&&) = delete;
-
-    Lock m_paging_lock { "VMObject" };
 };