mm/compaction: set compact_cached_free_pfn correctly in update_pageblock_skip

Patch series "Fixes and cleanups to compaction", v2.

This series contains random fixes and cleanups to free page isolation in
compaction.  This is based on another compact series[1].  More details can
be found in respective patches.


This patch (of 4):

We will set skip to page block of block_start_pfn, it's more reasonable to
set compact_cached_free_pfn to page block before the block_start_pfn.

Link: https://lkml.kernel.org/r/20230803094901.2915942-1-shikemeng@huaweicloud.com
Link: https://lkml.kernel.org/r/20230803094901.2915942-2-shikemeng@huaweicloud.com


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