mm/page-writeback.c: improve arithmetic divisions

mainline inclusion
from mainline-5.5-rc7
commit 0a5d1a7f
category: bugfix
bugzilla: 28317
CVE: NA

-------------------------------------------------
Use div64_ul() instead of do_div() if the divisor is unsigned long, to
avoid truncation to 32-bit on 64-bit platforms.

Link: http://lkml.kernel.org/r/20200102081442.8273-4-wenyang@linux.alibaba.com


Signed-off-by: Wen Yang <wenyang@linux.alibaba.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Qian Cai <cai@lca.pw>
Cc: Tejun Heo <tj@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
(cherry picked from commit 0a5d1a7f)
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Reviewed-by: Jing Xiangfeng <jingxiangfeng@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>