FreeBSD: Clean up zfsdev_close to match Linux

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

Approved by:	so
Security:	FreeBSD-EN-22:12.zfs