Fixes to the seq_file document

On Friday 2008-03-28 19:20, Jonathan Corbet wrote:
>commit 9756ccfda31b4c4544aa010aacf71b6672d668e8
>Date:   Fri Mar 28 11:19:56 2008 -0600
>
>    Add the seq_file documentation

patch on top:

  - add const qualifiers
  - remove void* casts
  - use proper specifier (%Ld is not valid)

Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Jan Engelhardt <jengelh@computergmbh.de>