mm/compaction: correct comment of cached migrate pfn update

Commit e380bebe ("mm, compaction: keep migration source private to a
single compaction instance") moved update of async and sync
compact_cached_migrate_pfn from update_pageblock_skip to
update_cached_migrate but left the comment behind.  Move the relevant
comment to correct this.

Link: https://lkml.kernel.org/r/20230804110454.2935878-6-shikemeng@huaweicloud.com


Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>