proc: cleanup: use seq_release_private() where appropriate

We can save some lines of code by using seq_release_private().

Signed-off-by: Martin Peschke <mp3@de.ibm.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>