Fix readonly check for vdev user properties

VDEV_PROP_USERPROP is equal do VDEV_PROP_INVAL and so is not a real
property. That's why vdev_prop_readonly() does not work right for
it. In particular it may declare all vdev user properties readonly
on FreeBSD.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Rob Norris <robn@despairlabs.com>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.
Closes #16890