Kernel: Use non-locking {Nonnull,}RefPtr for OpenFileDescription

This patch switches away from {Nonnull,}LockRefPtr to the non-locking
smart pointers throughout the kernel.

I've looked at the handful of places where these were being persisted
and I don't see any race situations.

Note that the process file descriptor table (Process::m_fds) was already
guarded via MutexProtected.

Kernel/FileSystem/FileBackedFileSystem.h

@@ -38,6 +38,6 @@ private:
     virtual bool is_file_backed() const override { return true; }
 
     IntrusiveListNode<FileBackedFileSystem> m_file_backed_file_system_node;
-    mutable NonnullLockRefPtr<OpenFileDescription> m_file_description;
+    NonnullRefPtr<OpenFileDescription> m_file_description;
 };
 }