mm, swap: merge zeromap into swap table

mm, swap: merge zeromap into swap table

By allocating one additional bit in the swap table entry's flags field
alongside the count, we can store the zeromap inline

For 64 bit systems, zeromap will store in the swap table, avoiding zeromap
allocation.  It reduces the allocated memory.  That is the happy path.

For certain 32-bit archs, there might not be enough bits in the swap table
to contain both PFN and flags.  Therefore, conditionally let each cluster
have a zeromap field at build time, and use that instead.  If the swapfile
cluster is not fully used, it will still save memory for zeromap.  The
empty cluster does not allocate a zeromap.  In the worst case, all cluster
are fully populated.  We will use memory similar to the previous zeromap
implementation.

A few macros were moved to different headers for build time struct
definition.

[akpm@linux-foundation.org: swap_cluster_alloc_table(): remove unused local `ret]
[akpm@linux-foundation.org: fix unused label `err_free']
Link: https://lore.kernel.org/20260517-swap-table-p4-v5-12-88ae43e064c7@tencent.com
Signed-off-by: Kairui Song <kasong@tencent.com>
Acked-by: Chris Li <chrisl@kernel.org>
Reviewed-by: Youngjun Park <youngjun.park@lge.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Baoquan He <bhe@redhat.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Chengming Zhou <chengming.zhou@linux.dev>
Cc: David Hildenbrand <david@kernel.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>