zram: do not leak page on writeback_store error path

Ensure the page used for local object data is freed on error out path.

Link: https://lkml.kernel.org/r/20250303022425.285971-19-senozhatsky@chromium.org
Fixes: 330edc2bc0 (zram: rework writeback target selection strategy)
Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Hillf Danton <hdanton@sina.com>
Cc: Kairui Song <ryncsn@gmail.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Yosry Ahmed <yosry.ahmed@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

drivers/block/zram/zram_drv.c

@@ -787,7 +787,7 @@ static ssize_t writeback_store(struct device *dev,
 	unsigned long index = 0;
 	struct bio bio;
 	struct bio_vec bio_vec;
-	struct page *page;
+	struct page *page = NULL;
 	ssize_t ret = len;
 	int mode, err;
 	unsigned long blk_idx = 0;
@@ -929,8 +929,10 @@ next:
 
 	if (blk_idx)
 		free_block_bdev(zram, blk_idx);
-	__free_page(page);
+
 release_init_lock:
+	if (page)
+		__free_page(page);
 	release_pp_ctl(zram, ctl);
 	atomic_set(&zram->pp_in_progress, 0);
 	up_read(&zram->init_lock);