Fix replication lag calculation in readwritesplit

The value used to represent the lack of a configured replication lag was
different than was used in other parts of MaxScale.

server/modules/routing/readwritesplit/rwsplit_select_backends.cc

@@ -265,7 +265,7 @@ RWBackend* RWSplitSession::get_slave_backend(int max_rlag)
             }
         }
 
-        if (max_rlag > 0)
+        if (max_rlag != SERVER::RLAG_UNDEFINED)
         {
             auto state = rlag_ok ? SERVER::RLagState::ABOVE_LIMIT : SERVER::RLagState::BELOW_LIMIT;
             backend->change_rlag_state(state, max_rlag);