Changed how we write out superblock to use append

Making the superblock look like "just another entry" allows us to treat
the superblock like "just another entry" and reuse a decent amount of
logic that would otherwise only be used a format and mount time. In this
case we can use append to write out the superblock like it was creating
a new entry on the filesystem.

tests/test_corrupt.sh

@@ -73,7 +73,7 @@ lfs_mktree
 lfs_chktree
 
 echo "--- Block corruption ---"
-for i in {0..33}
+for i in {2..33}
 do 
     rm -rf blocks
     mkdir blocks
@@ -83,7 +83,7 @@ do
 done
 
 echo "--- Block persistance ---"
-for i in {0..33}
+for i in {2..33}
 do 
     rm -rf blocks
     mkdir blocks