Kernel: Use default con/de-structors

This may seem like a no-op change, however it shrinks down the Kernel by a bit:
.text -432
.unmap_after_init -60
.data -480
.debug_info -673
.debug_aranges 8
.debug_ranges -232
.debug_line -558
.debug_str -308
.debug_frame -40

With '= default', the compiler can do more inlining, hence the savings.
I intentionally omitted some opportunities for '= default', because they
would increase the Kernel size.

Kernel/FileSystem/BlockBasedFileSystem.cpp

@@ -51,7 +51,7 @@ public:
         }
     }
 
-    ~DiskCache() { }
+    ~DiskCache() = default;
 
     bool is_dirty() const { return m_dirty; }
     void set_dirty(bool b) { m_dirty = b; }