FreeBSD: Fix scope of deadman tunables

A few deadman tunables ended up in the wrong sysctl node.

Move them to vfs.zfs.deadman.*

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #11715