Kernel: Sprinkle some lockers in Inode

It did look pretty suspicious the way we were accessing members in some
of these functions without taking the lock first.

Kernel/FileSystem/Inode.h

@@ -113,7 +113,7 @@ public:
     void register_watcher(Badge<InodeWatcher>, InodeWatcher&);
     void unregister_watcher(Badge<InodeWatcher>, InodeWatcher&);
 
-    FIFO& fifo();
+    NonnullRefPtr<FIFO> fifo();
 
     // For InlineLinkedListNode.
     Inode* m_next { nullptr };