nbd: Remove superfluous casts

In Linux kernel code it is preferred not to use a cast when converting a
void pointer to another pointer type.

Cc: Christoph Hellwig <hch@lst.de>
Cc: Josef Bacik <jbacik@fb.com>
Cc: Yu Kuai <yukuai3@huawei.com>
Cc: Markus Pargmann <mpa@pengutronix.de>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20240510202313.25209-3-bvanassche@acm.orgSigned-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/nbd.c

@@ -222,7 +222,7 @@ static ssize_t pid_show(struct device *dev,
 			struct device_attribute *attr, char *buf)
 {
 	struct gendisk *disk = dev_to_disk(dev);
-	struct nbd_device *nbd = (struct nbd_device *)disk->private_data;
+	struct nbd_device *nbd = disk->private_data;
 
 	return sprintf(buf, "%d\n", nbd->pid);
 }
@@ -236,7 +236,7 @@ static ssize_t backend_show(struct device *dev,
 		struct device_attribute *attr, char *buf)
 {
 	struct gendisk *disk = dev_to_disk(dev);
-	struct nbd_device *nbd = (struct nbd_device *)disk->private_data;
+	struct nbd_device *nbd = disk->private_data;
 
 	return sprintf(buf, "%s\n", nbd->backend ?: "");
 }