percpu: fix a mismatch between code and comment

When pcpu_build_alloc_info() searches best_upa value, it ignores current value
if the number of waste units exceeds 1/3 of the number of total cpus. But the
comment on the code says that it will ignore if wastage is over 25%.
Modify the comment.
Signed-off-by: Namhyung Kim <namhyung@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

mm/percpu.c

@@ -1164,7 +1164,7 @@ static struct pcpu_alloc_info * __init pcpu_build_alloc_info(
 		}
 
 		/*
-		 * Don't accept if wastage is over 25%.  The
+		 * Don't accept if wastage is over 1/3.  The
 		 * greater-than comparison ensures upa==1 always
 		 * passes the following check.
 		 */