afinet: max packet length for spoof source is set to 1024

If actual packet size is bigger than AFInetDestDriver.spoof_source_maxmsglen,
the packet is truncated.

AFInetDestDriver.spoof_source_maxmsglen is set to 1024
and cannot set via config file at this time.

Signed-off-by: Budai Laszlo <Laszlo.Budai@balabit.com>