JSON: fix user properties output for zfs list

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

Reviewed-by: Ameer Hamza <ahamza@ixsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Umer Saleem <usaleem@ixsystems.com>
Closes openzfs#16732

cmd/zfs/zfs_main.c

@@ -3761,8 +3761,13 @@ collect_dataset(zfs_handle_t *zhp, list_cbdata_t *cb)
 		if (cb->cb_json) {
 			if (pl->pl_prop == ZFS_PROP_NAME)
 				continue;
+			const char *prop_name;
+			if (pl->pl_prop != ZPROP_USERPROP)
+				prop_name = zfs_prop_to_name(pl->pl_prop);
+			else
+				prop_name = pl->pl_user_prop;
 			if (zprop_nvlist_one_property(
-			    zfs_prop_to_name(pl->pl_prop), propstr,
+			    prop_name, propstr,
 			    sourcetype, source, NULL, props,
 			    cb->cb_json_as_int) != 0)
 				nomem();