mm, swap: consolidate bad slots setup and make it more robust

mm, swap: consolidate bad slots setup and make it more robust

In preparation for using the swap table to track bad slots directly, move
the bad slot setup to one place, set up the swap_map mark, and cluster
counter update together.

While at it, provide more informative logs and a more robust fallback if
any bad slot info looks incorrect.

Fixes a potential issue that a malformed swap file may cause the cluster
to be unusable upon swapon, and provides a more verbose warning on a
malformed swap file

Link: https://lkml.kernel.org/r/20260218-swap-table-p3-v3-4-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>