ext4: fix comments to say "ext4"

Change second/third to fourth.

Signed-off-by: Shen Feng <shen@cn.fujitsu.com>
Signed-off-by: Mingming Cao <cmm@us.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
diff --git a/fs/ext4/ext4_i.h b/fs/ext4/ext4_i.h
index 26a4ae2..abf2744 100644
--- a/fs/ext4/ext4_i.h
+++ b/fs/ext4/ext4_i.h
@@ -79,7 +79,7 @@
 };
 
 /*
- * third extended file system inode data in memory
+ * fourth extended file system inode data in memory
  */
 struct ext4_inode_info {
 	__le32	i_data[15];	/* unconverted */