ublk: simplify ublk_ch_open and ublk_ch_release

fops->open and fops->release are always paired.  Use simple atomic bit
ops ot indicate if the device is opened instead of a count that can
only be 0 and 1 and a useless cmpxchg loop in ublk_ch_release.

Also don't bother clearing file->private_data is the file is about to
be freed anyway.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20220721130916.1869719-5-hch@lst.deSigned-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/ublk_drv.c

@@ -125,7 +125,8 @@ struct ublk_device {
 	struct cdev		cdev;
 	struct device		cdev_dev;
 
-	atomic_t		ch_open_cnt;
+#define UB_STATE_OPEN		(1 << 0)
+	unsigned long		state;
 	int			ub_number;
 
 	struct mutex		mutex;
@@ -647,21 +648,17 @@ static int ublk_ch_open(struct inode *inode, struct file *filp)
 	struct ublk_device *ub = container_of(inode->i_cdev,
 			struct ublk_device, cdev);
 
-	if (atomic_cmpxchg(&ub->ch_open_cnt, 0, 1) == 0) {
-		filp->private_data = ub;
-		return 0;
-	}
-	return -EBUSY;
+	if (test_and_set_bit(UB_STATE_OPEN, &ub->state))
+		return -EBUSY;
+	filp->private_data = ub;
+	return 0;
 }
 
 static int ublk_ch_release(struct inode *inode, struct file *filp)
 {
 	struct ublk_device *ub = filp->private_data;
 
-	while (atomic_cmpxchg(&ub->ch_open_cnt, 1, 0) != 1)
-		cpu_relax();
-
-	filp->private_data = NULL;
+	clear_bit(UB_STATE_OPEN, &ub->state);
 	return 0;
 }