Kernel: Fick infinite recursion when filling up disk cache

We can't be calling the virtual FS::flush_writes() in order to flush
the disk cache from within the disk cache, since an FS subclass may
try to do cache stuff in its flush_writes() implementation.

Instead, separate out the implementation of DiskBackedFS's flushing
logic into a flush_writes_impl() and call that from the cache code.

Kernel/FileSystem/DiskBackedFileSystem.h

@@ -16,6 +16,8 @@ public:
 
     virtual void flush_writes() override;
 
+    void flush_writes_impl();
+
 protected:
     explicit DiskBackedFS(NonnullRefPtr<DiskDevice>&&);