mm/vmscan: fix hwpoisoned large folio handling in shrink_folio_list

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

mm/vmscan: fix hwpoisoned large folio handling in shrink_folio_list

In shrink_folio_list(), the hwpoisoned folio may be large folio, which
can't be handled by unmap_poisoned_folio(). For THP, try_to_unmap_one()
must be passed with TTU_SPLIT_HUGE_PMD to split huge PMD first and then
retry. Without TTU_SPLIT_HUGE_PMD, we will trigger null-ptr deref of
pvmw.pte. Even we passed TTU_SPLIT_HUGE_PMD, we will trigger a
WARN_ON_ONCE due to the page isn't in swapcache.

Since UCE is rare in real world, and race with reclaimation is more rare,
just skipping the hwpoisoned large folio is enough. memory_failure() will
handle it if the UCE is triggered again.

This happens when memory reclaim for large folio races with
memory_failure(), and will lead to kernel panic. The race is as
follows:

cpu0 cpu1
shrink_folio_list memory_failure
TestSetPageHWPoison
unmap_poisoned_folio
--> trigger BUG_ON due to
unmap_poisoned_folio couldn't
handle large folio

[tujinjiang@huawei.com: add comment to unmap_poisoned_folio()]