FreeBSD: Clean up zfsdev_close to match Linux

Resolve some oddities in zfsdev_close() which could result in a
panic and were not present in the equivalent function for Linux.

• Remove unused definition ZFS_MIN_MINOR
• FreeBSD: Simplify zfsdev state destruction
• Assert zs_minor is valid in zfsdev_close
• Make locking around zfsdev state match Linux

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #11720