Duplicate what Zach Brown did for pr_debug in commit
8b2a1fd1

[akpm@linux-foundation.org: fix a couple of things which broke]
Signed-off-by: NDan Williams <dan.j.williams@intel.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>