mm, swap: mark bad slots in swap table directly

mm, swap: mark bad slots in swap table directly

In preparing the deprecating swap_map, mark bad slots in the swap table
too when setting SWAP_MAP_BAD in swap_map.  Also, refine the swap table
sanity check on freeing to adapt to the bad slots change.  For swapoff,
the bad slots count must match the cluster usage count, as nothing should
touch them, and they contribute to the cluster usage count on swapon.  For
ordinary swap table freeing, the swap table of clusters with bad slots
should never be freed since the cluster usage count never reaches zero.

Link: https://lkml.kernel.org/r/20260218-swap-table-p3-v3-7-f4e34be021a7@tencent.com
Signed-off-by: Kairui Song <kasong@tencent.com>
Acked-by: Chris Li <chrisl@kernel.org>
Cc: Baoquan He <bhe@redhat.com>
Cc: Barry Song <baohua@kernel.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kairui Song <ryncsn@gmail.com>
Cc: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: kernel test robot <lkp@intel.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Nhat Pham <nphamcs@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>