zsmalloc: add zpool_ops field to zs_pool to store evict handlers

This adds a new field to zs_pool to store evict handlers for writeback,
analogous to the zbud allocator.

Link: https://lkml.kernel.org/r/20221128191616.1261026-6-nphamcs@gmail.comSigned-off-by: Nhat Pham <nphamcs@gmail.com>
Acked-by: Minchan Kim <minchan@kernel.org>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Dan Streetman <ddstreet@ieee.org>
Cc: Nitin Gupta <ngupta@vflare.org>
Cc: Seth Jennings <sjenning@redhat.com>
Cc: Vitaly Wool <vitaly.wool@konsulko.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/zsmalloc.c

@@ -242,6 +242,8 @@ struct zs_pool {
 #ifdef CONFIG_ZPOOL
 	/* List tracking the zspages in LRU order by most recently added object */
 	struct list_head lru;
+	struct zpool *zpool;
+	const struct zpool_ops *zpool_ops;
 #endif
 
 #ifdef CONFIG_ZSMALLOC_STAT
@@ -382,7 +384,14 @@ static void *zs_zpool_create(const char *name, gfp_t gfp,
 	 * different contexts and its caller must provide a valid
 	 * gfp mask.
 	 */
-	return zs_create_pool(name);
+	struct zs_pool *pool = zs_create_pool(name);
+
+	if (pool) {
+		pool->zpool = zpool;
+		pool->zpool_ops = zpool_ops;
+	}
+
+	return pool;
 }
 
 static void zs_zpool_destroy(void *pool)