CVE-2024-42316: 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 vmpressurecalclevel()

evictfolios() uses a second pass to reclaim folios that have gone through page writeback and become clean before it finishes the first pass, since foliorotatereclaimable() cannot handle those folios due to the isolation.

The second pass tries to avoid potential double counting by deducting scancontrol->nrscanned. However, this can result in underflow of nrscanned, under a condition where shrinkfoliolist() does not increment nrscanned, i.e., when foliotrylock() fails.

The underflow can cause the divisor, i.e., scale=scanned+reclaimed in vmpressurecalclevel(), to become zero, resulting in the following crash:

[exception RIP: vmpressureworkfn+101] processonework at ffffffffa3313f2b

Since scancontrol->nrscanned has no established semantics, the potential double counting has minimal risks. Therefore, fix the problem by not deducting scancontrol->nrscanned in evictfolios().