JSON: fix user properties output for zpool list

This commit fixes JSON output for zpool list when user properties are
requested with -o flag. This case needed to be handled specifically
since zpool_prop_to_name does not return property name for user
properties, instead it is stored in pl->pl_user_prop.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Umer Saleem <usaleem@ixsystems.com>
Closes #16734