sd-bus: when credentials of the "org.freedesktop.DBus" service are queried return the bus owner's credentials

The bus driver service is always implemented by the owner of the bus,
hence let's shortcut the credential operation and use our cached data.
This makes sure things simply work, given that dbus itself doesn't
support GetConnectionSELinuxSecurityContext() on the bus driver name
itself.

Fixes: #6120

src/libsystemd/sd-bus/bus-control.c

@@ -926,6 +926,9 @@ _public_ int sd_bus_get_name_creds(
         if (streq(name, "org.freedesktop.DBus.Local"))
                 return -EINVAL;
 
+        if (streq(name, "org.freedesktop.DBus"))
+                return sd_bus_get_owner_creds(bus, mask, creds);
+
         if (!BUS_IS_OPEN(bus->state))
                 return -ENOTCONN;