mm/vmscan: Maintain TLB coherency in LRU code

As a workaround (and possibly a fix) for CPU spins observed on BCM2837,
use ptep_clear_flush_young instead of ptep_test_and_clear_young inside
lru_gen_look_around in order to expose PTE changes to the MMU. Note that
on architectures that don't require an explicit flush,
ptep_clear_flush_young just calls ptep_test_and_clear_young.

Signed-off-by: Phil Elwell <phil@raspberrypi.com>

mm/vmscan.c

@@ -4736,7 +4736,7 @@ void lru_gen_look_around(struct page_vma_mapped_walk *pvmw)
 		if (!folio)
 			continue;
 
-		if (!ptep_test_and_clear_young(vma, addr, pte + i))
+		if (!ptep_clear_flush_young(vma, addr, pte + i))
 			VM_WARN_ON_ONCE(true);
 
 		young++;