zfs: Fix zvol_cdev_open locking

First open locking changes were correctly applied to zvol_geom_open but
incorrectly applied to zvol_cdev_open, causing spa_namespace_lock to be
held indefinitely.

Make the first open locking in zvol_cdev_open match zvol_geom_open.

This change has been accepted upstream in openzfs/zfs#13016 but is not
yet merged.

Reviewed by:	mav
Fixes:		e92ffd9
Sponsored by:	iXsystems, Inc.

sys/contrib/openzfs/module/os/freebsd/zfs/zvol_os.c

@@ -928,8 +928,8 @@ zvol_cdev_open(struct cdev *dev, int flags, int fmt, struct thread *td)
 		 */
 		if (!mutex_owned(&spa_namespace_lock)) {
 			if (!mutex_tryenter(&spa_namespace_lock)) {
-				rw_exit(&zvol_state_lock);
-				mutex_enter(&spa_namespace_lock);
+				mutex_exit(&zv->zv_state_lock);
+				rw_exit(&zv->zv_suspend_lock);
 				kern_yield(PRI_USER);
 				goto retry;
 			} else {