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/Tasks/SyncTask.cpp

@@ -14,7 +14,7 @@ namespace Kernel {
 
 UNMAP_AFTER_INIT void SyncTask::spawn()
 {
-    RefPtr<Thread> syncd_thread;
+    LockRefPtr<Thread> syncd_thread;
     (void)Process::create_kernel_process(syncd_thread, KString::must_create("VFS Sync Task"sv), [] {
         dbgln("VFS SyncTask is running");
         for (;;) {