mm: vmscan: skip anon folios zswap would refuse when writeback is off

shrink_folio_list() 逐 folio 跳过 zswap 必拒的匿名页：writeback=0 且池满/超 zswap.max 时 swap_writeout() 只返回 ACTIVATE，避免反复分槽、unmap 却无回收。