mm, swap: implement dynamic allocation of swap table

Now swap table is cluster based, which means free clusters can free its
table since no one should modify it.

There could be speculative readers, like swap cache look up, protect them
by making them RCU protected.  All swap table should be filled with null
entries before free, so such readers will either see a NULL pointer or a
null filled table being lazy freed.

On allocation, allocate the table when a cluster is used by any order.

This way, we can reduce the memory usage of large swap device
significantly.

This idea to dynamically release unused swap cluster data was initially
suggested by Chris Li while proposing the cluster swap allocator and it
suits the swap table idea very well.

Link: https://lkml.kernel.org/r/20250916160100.31545-15-ryncsn@gmail.com
Co-developed-by: Chris Li <chrisl@kernel.org>
Signed-off-by: Chris Li <chrisl@kernel.org>
Signed-off-by: Kairui Song <kasong@tencent.com>
Suggested-by: Chris Li <chrisl@kernel.org>
Reviewed-by: Barry Song <baohua@kernel.org>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Baoquan He <bhe@redhat.com>
Cc: David Hildenbrand <david@redhat.com>
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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