ofproto-dpif-upcall: Forward packets in order of arrival.

ofproto-dpif-upcall: Forward packets in order of arrival.

Until now, the code in ofproto-dpif-upcall (and the code that preceded it
in ofproto-dpif) obtained a batch of incoming packets, inserted them into
a hash table based on hashes of their flows, processed them, and then
forwarded them in hash order.  Usually this maintains order within a single
network connection, but because OVS's notion of a flow is so fine-grained,
it can reorder packets within (e.g.) a TCP connection if two packets
handled in a single batch have (e.g.) different ECN values.

This commit fixes the problem by making ofproto-dpif-upcall always forward
packets in the same order they were received.

This is far from the minimal change necessary to avoid reordering packets.
I think that the code is easier to understand afterward.

Reported-by: Dmitry Fleytman <dfleytma@redhat.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Jarno Rajahalme <jrajahalme@nicira.com>