Kernel: If a VMObject is shared, broadcast page remappings

If we remap pages (e.g. lazy allocation) inside a VMObject that is
shared among more than one region, broadcast it to any other region
that may be mapping the same page.

Kernel/VM/VMObject.cpp

@@ -46,6 +46,7 @@ VMObject::VMObject(size_t size)
 VMObject::~VMObject()
 {
     MM.unregister_vmobject(*this);
+    ASSERT(m_regions_count.load(AK::MemoryOrder::memory_order_relaxed) == 0);
 }
 
 }