vswitch: Send gratuitous learning packets when bond slaves go down.

vswitch: Send gratuitous learning packets when bond slaves go down.

When a bond slave goes down, all of the MACs that were on it are migrated
to another slave, but this is not apparent to the switch that the bond is
connected to until each MAC sends out a packet.  This causes incoming
traffic for a given MAC to be dropped until the MAC sends out a packet.
This is not usually a problem, because traffic is not ordinarily one-way,
and we can't avoid losing some packets in some cases, but we can do a
little better by sending out a gratuitous learning packet on the new slave
as soon as we know about it, and that is what this commit implements.

Bug #1290.