tools/vm/page_owner_sort.c: use fprintf() to send error messages to stderr

Error messages should be send to stderr using fprintf() instead of
printf().

This work is coauthored by
        Yixuan Cao
        Shenghong Han
        Yinan Zhang
        Chongxi Zhao
        Yuhong Feng
        Yongqiang Liu

Link: https://lkml.kernel.org/r/20220401024856.767-1-yejiajian2018@email.szu.edu.cn


Signed-off-by: Jiajian Ye <yejiajian2018@email.szu.edu.cn>
Cc: Shenghong Han <hanshenghong2019@email.szu.edu.cn>
Cc: Yixuan Cao <caoyixuan2019@email.szu.edu.cn>
Cc: Yinan Zhang <zhangyinan2019@email.szu.edu.cn>
Cc: Chongxi Zhao <zhaochongxi2019@email.szu.edu.cn>
Cc: Yuhong Feng <yuhongf@szu.edu.cn>
Cc: Yongqiang Liu <liuyongqiang13@huawei.com>
Cc: Haowen Bai <baihaowen@meizu.com>
Cc: Sean Anderson <seanga2@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>