ofproto-dpif-xlate: Work around Linux netdev_max_backlog limit.

Linux has an internal queue that temporarily holds packets transmitted to
certain network devices.  If too many packets are transmitted to such
network devices within a single list of actions, then packets tend to get
dropped.  Broadcast or flooded or multicast packets on bridges with
thousands of ports are examples of how this can occur.

This commit avoids the problem by implementing a flow in userspace when it
outputs its packet more times than the maximum length of the queue.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Flavio Leitner <fbl@redhat.com>
Tested-by: Flavio Leitner <fbl@redhat.com>