mm/damon/core: prevent unnecessary overflow in damos_set_effective_quota()

On 32-bit systems, the throughput calculation in
damos_set_effective_quota() is prone to unnecessary multiplication
overflow.  Using mult_frac() to fix it.

Andrew Paniakin also recently found and privately reported this issue, on
64 bit systems.  This can also happen on 64-bit systems, once the charged
size exceeds ~17 TiB.  On systems running for long time in production,
this issue can actually happen.

More specifically, when a DAMOS scheme having the time quota run for
longtime, throughput calculation can overflow and set esz too small.  As a
result, speed of the scheme get unexpectedly slow.

Link: https://lkml.kernel.org/r/20250821125555.3020951-1-yanquanmin1@huawei.com


Fixes: 1cd24303 ("mm/damon/schemes: implement time quota")
Signed-off-by: Quanmin Yan <yanquanmin1@huawei.com>
Reported-by: Andrew Paniakin <apanyaki@amazon.com>
Reviewed-by: SeongJae Park <sj@kernel.org>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: ze zuo <zuoze1@huawei.com>
Cc: <stable@vger.kernel.org>	[5.16+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>