mm/shmem, swap: remove SWAP_MAP_SHMEM

mm/shmem, swap: remove SWAP_MAP_SHMEM

The SWAP_MAP_SHMEM state was introduced in the commit aaa468653b4a
("swap_info: note SWAP_MAP_SHMEM"), to quickly determine if a swap entry
belongs to shmem during swapoff.

However, swapoff has since been rewritten in the commit b56a2d8af914 ("mm:
rid swapoff of quadratic complexity").  Now having swap count ==
SWAP_MAP_SHMEM value is basically the same as having swap count == 1, and
swap_shmem_alloc() behaves analogously to swap_duplicate().  The only
difference of note is that swap_shmem_alloc() does not check for -ENOMEM
returned from __swap_duplicate(), but it is OK because shmem never
re-duplicates any swap entry it owns.  This will stil be safe if we use
(batched) swap_duplicate() instead.

This commit adds swap_duplicate_nr(), the batched variant of
swap_duplicate(), and removes the SWAP_MAP_SHMEM state and the associated
swap_shmem_alloc() helper to simplify the state machine (both mentally and
in terms of actual code).  We will also have an extra state/special value
that can be repurposed (for swap entries that never gets re-duplicated).

Link: https://lkml.kernel.org/r/20251220-swap-table-p2-v5-8-8862a265a033@tencent.com
Signed-off-by: Kairui Song <kasong@tencent.com>
Signed-off-by: Nhat Pham <nphamcs@gmail.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Tested-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Baoquan He <bhe@redhat.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Chris Li <chrisl@kernel.org>
Cc: Rafael J. Wysocki (Intel) <rafael@kernel.org>
Cc: Yosry Ahmed <yosry.ahmed@linux.dev>
Cc: Deepanshu Kartikey <kartikey406@gmail.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kairui Song <ryncsn@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>