mm/zsmalloc: clarify class per-fullness zspage counts

We always use insert_zspage() and remove_zspage() to update zspage's
fullness location, which will account correctly.

But this special async free path use "splice" instead of remove_zspage(),
so the per-fullness zspage count for ZS_INUSE_RATIO_0 won't decrease.

Clean things up by decreasing when iterate over the zspage free list.

This doesn't actually fix anything.  ZS_INUSE_RATIO_0 is just a
"placeholder" which is never used anywhere.

Link: https://lkml.kernel.org/r/20240627075959.611783-1-chengming.zhou@linux.dev
Signed-off-by: Chengming Zhou <chengming.zhou@linux.dev>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Sergey Senozhatsky <senozhatsky@chromium.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/zsmalloc.c

@@ -1883,6 +1883,7 @@ static void async_free_zspage(struct work_struct *work)
 
 		class = zspage_class(pool, zspage);
 		spin_lock(&class->lock);
+		class_stat_dec(class, ZS_INUSE_RATIO_0, 1);
 		__free_zspage(pool, class, zspage);
 		spin_unlock(&class->lock);
 	}