Fix annoying bug that lead to dropped packets in the P4 system

Fix annoying bug that lead to dropped packets in the P4 system

dev/pktfifo.cc:
    Make it so that we don't unserialize the size of the fifo, clobering the fact that we were trying to make it bigger, and leading to a misleading config.out that states the un-overwritten max_size.
    Perhaps this should instead be a panic if the size (amount that was serialized) is bigger than the maxsize that was assigned by the configuration file.

--HG--
extra : convert_revision : d4b0527bfd7a584554ddc87c9b2103f7a3a72332