nilfs2: clean up sketch file

The sketch file is a file to mark checkpoints with user data.  It was
experimentally introduced in the original implementation, and now
obsolete.  The file was handled differently with regular files; the file
size got truncated when a checkpoint was created.

This stops the special treatment and will treat it as a regular file.
Most users are not affected because mkfs.nilfs2 no longer makes this file.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/filesystems/nilfs2.txt

@@ -161,8 +161,6 @@ the following meta data files:
  4) Data address translation file  -- Maps virtual block numbers to usual
     (DAT)                             block numbers.  This file serves to
                                       make on-disk blocks relocatable.
- 5) Sketch file (sketch)           -- Keeps read-only data which can be
-                                      associated with checkpoints (optional)
 
 The following figure shows a typical organization of the logs: