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/init.cpp

@@ -292,7 +292,7 @@ void init_stage2()
             }
         }
     }
-    auto e2fs = Ext2FS::create(*FileDescription::create(root_dev));
+    auto e2fs = Ext2FS::create(root_dev);
     if (!e2fs->initialize()) {
         klog() << "init_stage2: couldn't open root filesystem";
         hang();