x86, kmemcheck: Use KERN_WARNING for error reporting

As suggested by Vegard Nossum, use KERN_WARNING for error
reporting to make sure kmemcheck reports end up in syslog.

Suggested-by: Vegard Nossum <vegard.nossum@gmail.com>
Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>
Cc: Andrew Morton <akpm@linux-foundation.org>
LKML-Reference: <1261990935.4641.7.camel@penberg-laptop>
Signed-off-by: Ingo Molnar <mingo@elte.hu>