zsmalloc: reset compaction source zspage pointer after putback_zspage()

The current implementation of the compaction loop fails to set the source
zspage pointer to NULL in all cases, leading to a potential issue where
__zs_compact() could use a stale zspage pointer.  This pointer could even
point to a previously freed zspage, causing unexpected behavior in the
putback_zspage() and migrate_write_unlock() functions after returning from
the compaction loop.

Address the issue by ensuring that the source zspage pointer is always set
to NULL when it should be.

Link: https://lkml.kernel.org/r/20230417130850.1784777-1-senozhatsky@chromium.org
Fixes: 5a845e9f ("zsmalloc: rework compaction algorithm")
Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Reported-by: Yu Zhao <yuzhao@google.com>
Tested-by: Yu Zhao <yuzhao@google.com>
Reviewed-by: Yosry Ahmed <yosryahmed@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/zsmalloc.c

@@ -2239,8 +2239,8 @@ static unsigned long __zs_compact(struct zs_pool *pool,
 		if (fg == ZS_INUSE_RATIO_0) {
 			free_zspage(pool, class, src_zspage);
 			pages_freed += class->pages_per_zspage;
-			src_zspage = NULL;
 		}
+		src_zspage = NULL;
 
 		if (get_fullness_group(class, dst_zspage) == ZS_INUSE_RATIO_100
 		    || spin_is_contended(&pool->lock)) {