Kernel: Implement a simpler, bigger cache for DiskBackedFS

The hashmap cache was ridiculously slow and hurt us more than it helped
us. This patch replaces it with a flat memory cache that keeps up to
10'000 blocks in cache with a simple dirty bit.

The syncd task will wake up periodically and call flush_writes() on all
file systems, which now causes us to traverse the cache and write all
dirty blocks to disk.

There's a ton of room for improvement here, but this itself is already
drastically better when doing repeated GCC invocations.

Kernel/FileSystem/DiskBackedFileSystem.h

@@ -3,6 +3,8 @@
 #include "FileSystem.h"
 #include <AK/ByteBuffer.h>
 
+class DiskCache;
+
 class DiskBackedFS : public FS {
 public:
     virtual ~DiskBackedFS() override;
@@ -24,6 +26,8 @@ protected:
     bool write_blocks(unsigned index, unsigned count, const ByteBuffer&);
 
 private:
+    DiskCache& cache() const;
+
     NonnullRefPtr<DiskDevice> m_device;
-    HashMap<unsigned, ByteBuffer> m_write_cache;
+    mutable OwnPtr<DiskCache> m_cache;
 };