mm, swap: implement helpers for reserving data in the swap table

mm, swap: implement helpers for reserving data in the swap table

To prepare for using the swap table as the unified swap layer, introduce
macros and helpers for storing multiple kinds of data in a swap table
entry.

From now on, we are storing PFN in the swap table to make space for extra
counting bits (SWAP_COUNT).  Shadows are still stored as they are, as the
SWAP_COUNT is not used yet.

Also, rename shadow_swp_to_tb to shadow_to_swp_tb.  That's a spelling
error, not really worth a separate fix.

No behaviour change yet, just prepare the API.

Link: https://lkml.kernel.org/r/20260218-swap-table-p3-v3-6-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>