mm/pageblock-flags: remove PB_migratetype_bits/PB_migrate_end

mm/pageblock-flags: remove PB_migratetype_bits/PB_migrate_end

enum pageblock_bits defines the meaning of pageblock bits.  Currently
PB_migratetype_bits says the lowest 3 bits represents migratetype and
PB_migrate_end/MIGRATETYPE_MASK's definition rely on it with magical
computation.

Remove the definition of PB_migratetype_bits/PB_migrate_end.  Use
PB_migrate_[0|1|2] to represent lowest bits for migratetype.  Then we can
simplify related definition.

Also, MIGRATETYPE_AND_ISO_MASK is MIGRATETYPE_MASK add isolation bit.  Use
MIGRATETYPE_MASK in the definition of MIGRATETYPE_AND_ISO_MASK looks
cleaner.

No functional change intended.

Link: https://lkml.kernel.org/r/20250827070105.16864-3-richard.weiyang@gmail.com
Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
Suggested-by: David Hildenbrand <david@redhat.com>
Acked-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>