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.comSigned-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>

mm/compaction.c

@@ -469,6 +469,7 @@ static void update_cached_migrate(struct compact_control *cc, unsigned long pfn)
 
 	pfn = pageblock_end_pfn(pfn);
 
+	/* Update where async and sync compaction should restart */
 	if (pfn > zone->compact_cached_migrate_pfn[0])
 		zone->compact_cached_migrate_pfn[0] = pfn;
 	if (cc->mode != MIGRATE_ASYNC &&
@@ -490,7 +491,6 @@ static void update_pageblock_skip(struct compact_control *cc,
 
 	set_pageblock_skip(page);
 
-	/* Update where async and sync compaction should restart */
 	if (pfn < zone->compact_cached_free_pfn)
 		zone->compact_cached_free_pfn = pfn;
 }