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: e92ffd9b6268
Sponsored by: iXsystems, Inc.