Kernel: Rename Lock to Mutex

Let's be explicit about what kind of lock this is meant to be.

Kernel/FileSystem/BlockBasedFileSystem.h

@@ -42,7 +42,7 @@ private:
     DiskCache& cache() const;
     void flush_specific_block_if_needed(BlockIndex index);
 
-    mutable Lock m_cache_lock;
+    mutable Mutex m_cache_lock;
     mutable OwnPtr<DiskCache> m_cache;
 };