FreeBSD: add knlist_init_sx() for exclusive locks
This will be used to implement kqfilter support for zvol cdevs.
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Rob Wing <rew@FreeBSD.org>
Closes #13773