impl-node: run recovery cycle immediately

There is no reason to use the fd to schedule process_node, we can call
it directly when we detect an underrun need to complete the cycle with
whatever data made it to the driver.

This avoids starting the graph while the process_node is running and
avoids some stuttering.

See #3937

src/pipewire/impl-node.c

@@ -1871,7 +1871,7 @@ static int node_ready(void *data, int status)
 					" pending %d/%d", node->name, node->info.id,
 					state, a->position.clock.duration,
 					pending, state->required);
-			wake_target(&node->rt.target, nsec);
+			process_node(node);
 			check_states(node, nsec);
 			pw_impl_node_rt_emit_incomplete(node);
 		}