zram: fix NULL check before some freeing functions is not needed

fixed the below warning:
/drivers/block/zram/zram_drv.c:534:2-8: WARNING: NULL check
before some freeing functions is not needed.
Signed-off-by: Tian Tao <tiantao6@hisilicon.com>
Acked-by: Minchan Kim <minchan@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/zram/zram_drv.c

@@ -530,8 +530,7 @@ static ssize_t backing_dev_store(struct device *dev,
 
 	return len;
 out:
-	if (bitmap)
-		kvfree(bitmap);
+	kvfree(bitmap);
 
 	if (bdev)
 		blkdev_put(bdev, FMODE_READ | FMODE_WRITE | FMODE_EXCL);