mm, swap: use an enum to define all cluster flags and wrap flags changes

Currently, we are only using flags to indicate which list the cluster is
on.  Using one bit for each list type might be a waste, as the list type
grows, we will consume too many bits.  Additionally, the current mixed
usage of '&' and '==' is a bit confusing.

Make it clean by using an enum to define all possible cluster statuses. 
Only an off-list cluster will have the NONE (0) flag.  And use a wrapper
to annotate and sanitize all flag settings and list movements.

Link: https://lkml.kernel.org/r/20250113175732.48099-9-ryncsn@gmail.com


Signed-off-by: Kairui Song <kasong@tencent.com>
Suggested-by: Chris Li <chrisl@kernel.org>
Cc: Baoquan He <bhe@redhat.com>
Cc: Barry Song <v-songbaohua@oppo.com>
Cc: "Huang, Ying" <ying.huang@linux.alibaba.com>
Cc: Hugh Dickens <hughd@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kalesh Singh <kaleshsingh@google.com>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Yosry Ahmed <yosryahmed@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>