FreeBSD: add kqfilter support for zvol cdev

The only event hooked up is NOTE_ATTRIB, which is triggered when the
device is resized.

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Rob Wing <rew@FreeBSD.org>
Closes #13773