Correct checking bdev_check_media_change message

We're not looking for bdev_disk_changed.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #12492