lib/debugobjects.c: convert printk(KERN_DEBUG to pr_debug

Direct conversion of one KERN_DEBUG message without DEBUG definition
(suggested by Josh Triplett)

That message will now be disabled by default.  (see
Documentation/CodingStyle Chapter 13)

Signed-off-by: Fabian Frederick <fabf@skynet.be>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>