mm/page_isolation: make page isolation a standalone bit

During page isolation, the original migratetype is overwritten, since
MIGRATE_* are enums and stored in pageblock bitmaps.  Change
MIGRATE_ISOLATE to be stored a standalone bit, PB_migrate_isolate, like
PB_compact_skip, so that migratetype is not lost during pageblock
isolation.

Link: https://lkml.kernel.org/r/20250617021115.2331563-3-ziy@nvidia.com


Signed-off-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Brendan Jackman <jackmanb@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kirill A. Shuemov <kirill.shutemov@linux.intel.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Richard Chang <richardycc@google.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>