Check l2cache vdevs pending list inside the vdev_inuse()

The l2cache device could be added twice because vdev_inuse() does not
check spa_l2cache for added devices. Make l2cache vdevs inuse checking
logic more closer to spare vdevs.

Reviewed-by: George Amanakis <gamanakis@gmail.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Fedor Uporov <fuporov.vstack@gmail.com>
Closes #9153
Closes #12689