zram: fix invalid memory access

[1] tried to fix invalid memory access on zram->disk but it didn't
fix properly because get_disk failed during module exit path.

Actually, we don't need to reset zram->disk's capacity to zero
in module exit path so that this patch introduces new argument
"reset_capacity" on zram_reset_divice and it only reset it when
reset_store is called.

[1] 6030ea9,  zram: avoid invalid memory access in zram_exit()

Cc: Nitin Gupta <ngupta@vflare.org>
Cc: Jiang Liu <jiang.liu@huawei.com>
Cc: stable@vger.kernel.org
Signed-off-by: Minchan Kim <minchan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/zram/zram_drv.c

@@ -523,7 +523,7 @@ static int zram_bvec_rw(struct zram *zram, struct bio_vec *bvec, u32 index,
 	return ret;
 }
 
-static void zram_reset_device(struct zram *zram)
+static void zram_reset_device(struct zram *zram, bool reset_capacity)
 {
 	size_t index;
 	struct zram_meta *meta;
@@ -552,7 +552,8 @@ static void zram_reset_device(struct zram *zram)
 	memset(&zram->stats, 0, sizeof(zram->stats));
 
 	zram->disksize = 0;
-	set_capacity(zram->disk, 0);
+	if (reset_capacity)
+		set_capacity(zram->disk, 0);
 	up_write(&zram->init_lock);
 }
 
@@ -636,7 +637,7 @@ static ssize_t reset_store(struct device *dev,
 	if (bdev)
 		fsync_bdev(bdev);
 
-	zram_reset_device(zram);
+	zram_reset_device(zram, true);
 	return len;
 }
 
@@ -903,10 +904,12 @@ static void __exit zram_exit(void)
 	for (i = 0; i < num_devices; i++) {
 		zram = &zram_devices[i];
 
-		get_disk(zram->disk);
 		destroy_device(zram);
-		zram_reset_device(zram);
-		put_disk(zram->disk);
+		/*
+		 * Shouldn't access zram->disk after destroy_device
+		 * because destroy_device already released zram->disk.
+		 */
+		zram_reset_device(zram, false);
 	}
 
 	unregister_blkdev(zram_major, "zram");