ofproto: Reorganize in preparation for direct dpdk upcalls.

ofproto: Reorganize in preparation for direct dpdk upcalls.

This patch reorganizes ofproto-dpif in preparation for future patches
which allow direct upcall processing from dpif-netdev.  The main goals
are to share as much code as possible between the dpif-linux and
dpif-netdev upcall paths.  Additionally, to avoid confusing the
dpif-netdev fast path, the packet processing path should treat packets
and struct flow's as const.

Signed-off-by: Ethan Jackson <ethan@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>