mm, swap: mark swap address space ro and add context debug check

Swap cache is now backed by swap table, and the address space is not
holding any mutable data anymore.  And swap cache is now protected by the
swap cluster lock, instead of the XArray lock.  All access to swap cache
are wrapped by swap cache helpers.  Locking is mostly handled internally
by swap cache helpers, only a few __swap_cache_* helpers require the
caller to lock the cluster by themselves.

Worth noting that, unlike XArray, the cluster lock is not IRQ safe.  The
swap cache was very different compared to filemap, and now it's completely
separated from filemap.  Nothing wants to mark or change anything or do a
writeback callback in IRQ.

So explicitly document this and add a debug check to avoid further
potential misuse.  And mark the swap cache space as read-only to avoid any
user wrongly mixing unexpected filemap helpers with swap cache.

Link: https://lkml.kernel.org/r/20250916160100.31545-13-ryncsn@gmail.com
Signed-off-by: Kairui Song <kasong@tencent.com>
Acked-by: Chris Li <chrisl@kernel.org>
Acked-by: David Hildenbrand <david@redhat.com>
Suggested-by: Chris Li <chrisl@kernel.org>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Baoquan He <bhe@redhat.com>
Cc: Barry Song <baohua@kernel.org>
Cc: "Huang, Ying" <ying.huang@linux.alibaba.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: kernel test robot <oliver.sang@intel.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Yosry Ahmed <yosryahmed@google.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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