mm/mglru: avoid reclaim type fall back when isolation makes no progress

While isolation makes no progress in scan_folios(), we quickly fall back
to the other type in isolate_folios().  This is incorrect, as the current
type may still have sufficient folios.  Falling back can undermine the
positive_ctrl_err() result from get_type_to_scan(), which is derived from
swappiness.

So just continue scanning this type for another round.

Worth noting if the cold generations are all reclaimed, scan will no
longer make any progress either, which may undermine the swappiness again.
This is not a new issue and hence better be fixed later [1].

Link: https://lore.kernel.org/linux-mm/CAGsJ_4zjdOYEtuO6gNjABm7NDxW0skzBFNRNee-k2D6VwsYEQA@mail.gmail.com/ [1]
Link: https://lore.kernel.org/20260428-mglru-reclaim-v7-6-02fabb92dc43@tencent.com
Signed-off-by: Barry Song (Xiaomi) <baohua@kernel.org>
Signed-off-by: Kairui Song <kasong@tencent.com>
Reviewed-by: Kairui Song <kasong@tencent.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Chen Ridong <chenridong@huaweicloud.com>
Cc: Chris Li <chrisl@kernel.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: David Stevens <stevensd@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kalesh Singh <kaleshsingh@google.com>
Cc: Leno Hou <lenohou@gmail.com>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vernon Yang <vernon2gm@gmail.com>
Cc: Wei Xu <weixugc@google.com>
Cc: Yafang <laoar.shao@gmail.com>
Cc: Yuanchu Xie <yuanchu@google.com>
Cc: Yu Zhao <yuzhao@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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