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.cpp

@@ -50,7 +50,9 @@ public:
         }
         if (!oldest_clean_entry) {
             // Not a single clean entry! Flush writes and try again.
-            m_fs.flush_writes();
+            // NOTE: We want to make sure we only call DiskBackedFS flush here,
+            //       not some DiskBackedFS subclass flush!
+            m_fs.flush_writes_impl();
             return get(block_index);
         }
 
@@ -148,7 +150,7 @@ bool DiskBackedFS::read_blocks(unsigned index, unsigned count, u8* buffer) const
     return true;
 }
 
-void DiskBackedFS::flush_writes()
+void DiskBackedFS::flush_writes_impl()
 {
     LOCKER(m_lock);
     if (!cache().is_dirty())
@@ -163,8 +165,12 @@ void DiskBackedFS::flush_writes()
         entry.is_dirty = false;
     });
     cache().set_dirty(false);
-    dbg() << class_name() << ": "
-          << "Flushed " << count << " blocks to disk";
+    dbg() << class_name() << ": Flushed " << count << " blocks to disk";
+}
+
+void DiskBackedFS::flush_writes()
+{
+    flush_writes_impl();
 }
 
 DiskCache& DiskBackedFS::cache() const

Kernel/FileSystem/DiskBackedFileSystem.h

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