FreeBSD: Replace legacy make_dev() interface usage

The function make_dev_s() was introduced to replace make_dev() in
FreeBSD 11.0. It allows further specification of properties and flags
and returns an error code on failure. Using this we can fail loading
the module more gracefully than a panic in situations such as when a
device named zfs already exists. We already use it for zvols.

Use make_dev_s() for /dev/zfs.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #13854