mm/workingset: leave highest bits empty for anon shadow

Swap table entry will need 4 bits reserved for swap count in the shadow,
so the anon shadow should have its leading 4 bits remain 0.

This should be OK for the foreseeable future. Take 52 bits of physical
address space as an example: for 4K pages, there would be at most 40 bits
for addressable pages. Currently, we have 36 bits available (64 - 1 - 16
- 10 - 1, where XA_VALUE takes 1 bit for marker, MEM_CGROUP_ID_SHIFT takes
16 bits, NODES_SHIFT takes <=10 bits, WORKINGSET flags takes 1 bit).

So in the worst case, we previously need to pack the 40 bits of address in
36 bits fields using a 64K bucket (bucket_order = 4). After this, the
bucket will be increased to 1M. Which should be fine, as on such large
machines, the working set size will be way larger than the bucket size.

And for MGLRU's gen number tracking, it should be even more than enough,
MGLRU's gen number (max_seq) increment is much slower compared to the
eviction counter (nonresident_age).

And after all, either the refault distance or the gen distance is only a
hint that can tolerate inaccuracy just fine.

And the 4 bits can be shrunk to 3, or extended to a higher value if needed
later.

Link: https://lkml.kernel.org/r/20260218-swap-table-p3-v3-5-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>