Kernel: Make self-contained locking smart pointers their own classes

Until now, our kernel has reimplemented a number of AK classes to
provide automatic internal locking:

- RefPtr
- NonnullRefPtr
- WeakPtr
- Weakable

This patch renames the Kernel classes so that they can coexist with
the original AK classes:

- RefPtr => LockRefPtr
- NonnullRefPtr => NonnullLockRefPtr
- WeakPtr => LockWeakPtr
- Weakable => LockWeakable

The goal here is to eventually get rid of the Lock* classes in favor of
using external locking.

Kernel/Arch/x86/common/PageDirectory.cpp

@@ -30,7 +30,7 @@ void PageDirectory::deregister_page_directory(PageDirectory* directory)
     cr3_map().remove(directory->cr3());
 }
 
-RefPtr<PageDirectory> PageDirectory::find_current()
+LockRefPtr<PageDirectory> PageDirectory::find_current()
 {
     SpinlockLocker lock(s_mm_lock);
     return cr3_map().find(read_cr3());