Kernel: Fix munmap() bad splitting of already-split Regions

When splitting an Region that's already the result of an earlier split,
we have to take the Region's offset-in-VMObject into account since it
may be non-zero.

Kernel/VM/Region.h

@@ -79,6 +79,11 @@ public:
         return size() / PAGE_SIZE;
     }
 
+    size_t offset_in_vmobject() const
+    {
+        return m_offset_in_vmo;
+    }
+
     int commit();
 
     size_t amount_resident() const;