mm/mglru: fix div-by-zero in vmpressure_calc_level()

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

mm/mglru: fix div-by-zero in vmpressure_calc_level()

evict_folios() uses a second pass to reclaim folios that have gone through

page writeback and become clean before it finishes the first pass, since

folio_rotate_reclaimable() cannot handle those folios due to the

isolation.

The second pass tries to avoid potential double counting by deducting

scan_control->nr_scanned. However, this can result in underflow of

nr_scanned, under a condition where shrink_folio_list() does not increment

nr_scanned, i.e., when folio_trylock() fails.

The underflow can cause the divisor, i.e., scale=scanned+reclaimed in

vmpressure_calc_level(), to become zero, resulting in the following crash:

[exception RIP: vmpressure_work_fn+101]

process_one_work at ffffffffa3313f2b

Since scan_control->nr_scanned has no established semantics, the potential

double counting has minimal risks. Therefore, fix the problem by not

deducting scan_control->nr_scanned in evict_folios().