zvol_first_open can fail with EINTR if spa_namespace_lock is not held
and cannot be taken without waiting.
Apply the same logic that was done for zvol_geom_open to take
spa_namespace_lock if not already held on first open in zvol_cdev_open.
Reviewed-by: Matt Macy <mmacy@FreeBSD.org>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes#11175
4a2e9811e9