Revert "Kernel: Change Ext2FS to be backed by a file instead of a block device"

This reverts commit 6b59311d4b.

Reverting these changes since they broke things.
Fixes #1608.

Kernel/FileSystem/FileSystem.h

@@ -87,7 +87,7 @@ public:
 
     int block_size() const { return m_block_size; }
 
-    virtual bool is_file_backed() const { return false; }
+    virtual bool is_disk_backed() const { return false; }
 
 protected:
     FS();