Kernel: Regions should be mapped into a PageDirectory, not a Process

This patch changes the parameter to Region::map() to be a PageDirectory
since that matches how we think about the memory model:

Regions are views onto VMObjects, and are mapped into PageDirectories.
Each Process has a PageDirectory. The kernel also has a PageDirectory.

Kernel/VM/Region.cpp

@@ -216,7 +216,7 @@ void Region::unmap(ShouldDeallocateVirtualMemoryRange deallocate_range)
     release_page_directory();
 }
 
-void Region::map(Process& process)
+void Region::map(PageDirectory& page_directory)
 {
-    MM.map_region_at_address(process.page_directory(), *this, vaddr());
+    MM.map_region_at_address(page_directory, *this, vaddr());
 }