mm/kmemleak-test.c: use pr_fmt for logging

Signed-off-by: Fabian Frederick <fabf@skynet.be>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>