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.cpp

@@ -0,0 +1,41 @@
+#include <Kernel/VM/PurgeableVMObject.h>
+#include <Kernel/VM/PhysicalPage.h>
+
+NonnullRefPtr<PurgeableVMObject> PurgeableVMObject::create_with_size(size_t size)
+{
+    return adopt(*new PurgeableVMObject(size));
+}
+
+PurgeableVMObject::PurgeableVMObject(size_t size)
+    : AnonymousVMObject(size)
+{
+}
+
+PurgeableVMObject::PurgeableVMObject(const PurgeableVMObject& other)
+    : AnonymousVMObject(other)
+{
+}
+
+PurgeableVMObject::~PurgeableVMObject()
+{
+}
+
+NonnullRefPtr<VMObject> PurgeableVMObject::clone()
+{
+    return adopt(*new PurgeableVMObject(*this));
+}
+
+int PurgeableVMObject::purge()
+{
+    LOCKER(m_paging_lock);
+    if (!m_volatile)
+        return 0;
+    int purged_page_count = 0;
+    for (size_t i = 0; i < m_physical_pages.size(); ++i) {
+        if (m_physical_pages[i])
+            ++purged_page_count;
+        m_physical_pages[i] = nullptr;
+    }
+    m_was_purged = true;
+    return purged_page_count;
+}