sequence: immediately stop child on exit

The sequence app should immediately stop the child when it called
parent().exit(). Otherwise, the child will continue execution which
causes a race condition: The child's ld.lib.so will eventually destruct
an Attached_rom_dataspace for the config rom. If sequence destructed the
corresponding service first, we will get an Ipc_error.

genodelabs/genode#4267

repos/os/src/app/sequence/main.cc

@@ -150,6 +150,7 @@ struct Sequence::Child : Genode::Child_policy
 	void exit(int exit_value) override
 	{
 		_exit_value = exit_value;
+		_child.close_all_sessions();
 		_exit_transmitter.submit();
 	}