mm/page_alloc: use xxx_pageblock_isolate() for better reading

Patch series "mm/pageblock: improve readability of some pageblock
handling", v3.

During code reading, found two possible points to improve the readability
of pageblock handling.

Patch 1: isolate bit is standalone and there are dedicated helpers.
Instead of check the bit directly, we could use the helper to do it.

Patch 2: remove PB_migratetype_bits and PB_migrate_end to reduce magical
computation.


This patch (of 2):

Since commit e904bce2d9d4 ("mm/page_isolation: make page isolation a
standalone bit"), it provides dedicated helper to handle isolation.

Change to use these helpers to be better reading.

No functional change intended.

Link: https://lkml.kernel.org/r/20250827070105.16864-1-richard.weiyang@gmail.com
Link: https://lkml.kernel.org/r/20250827070105.16864-2-richard.weiyang@gmail.com
Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
Acked-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: David Hildenbrand <david@redhat.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>