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 openzfs#16734

Author: Umer Saleem

cmd/zpool/zpool_main.c

@@ -6882,8 +6882,13 @@ collect_pool(zpool_handle_t *zhp, list_cbdata_t *cb)
 		if (cb->cb_json) {
 			if (pl->pl_prop == ZPOOL_PROP_NAME)
 				continue;
+			const char *prop_name;
+			if (pl->pl_prop != ZPROP_USERPROP)
+				prop_name = zpool_prop_to_name(pl->pl_prop);
+			else
+				prop_name = pl->pl_user_prop;
 			(void) zprop_nvlist_one_property(
-			    zpool_prop_to_name(pl->pl_prop), propstr,
+			    prop_name, propstr,
 			    sourcetype, NULL, NULL, props, cb->cb_json_as_int);
 		} else {
 			/*