Revert "mm/writeback: fix possible divide-by-zero in wb_dirty_limits(), again"

In the Linux kernel, the following vulnerability has been resolved:

Revert "mm/writeback: fix possible divide-by-zero in wb_dirty_limits(), again"

Patch series "mm: Avoid possible overflows in dirty throttling".

Dirty throttling logic assumes dirty limits in page units fit into

32-bits. This patch series makes sure this is true (see patch 2/2 for

more details).

This patch (of 2):

This reverts commit 9319b647902cbd5cc884ac08a8a6d54ce111fc78.

The commit is broken in several ways. Firstly, the removed (u64) cast

from the multiplication will introduce a multiplication overflow on 32-bit

archs if wb_thresh * bg_thresh >= 1<<32 (which is actually common - the

default settings with 4GB of RAM will trigger this). Secondly, the

div64_u64() is unnecessarily expensive on 32-bit archs. We have

div64_ul() in case we want to be safe & cheap. Thirdly, if dirty

thresholds are larger than 1<<32 pages, then dirty balancing is going to

blow up in many other spectacular ways anyway so trying to fix one

possible overflow is just moot.