ofproto/bond: simplify rebalancing logic

ofproto/bond: simplify rebalancing logic

The current bond relancing logic is more complicated than necessary.
When considering a bucket for rebalancing, we just need to make sure
post rebalancing traffic will be closer to the ideal traffic split
than before. This patch implements the simplification.

There is a bug is current algorithm that causes a heavyly loaded bucket
to ping-pong for each reblancing interval. The simplied loigc also fixes
this bug.

Though not the main motivation for the change, computations are now
done with integer math rather than floating math.

Reported-by: Gregory Smith <gasmith@nutanix.com>
tested-by: Gregory Smith <gasmith@nutanix.com>
Signed-off-by: Andy Zhou <azhou@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>