[BRIDGE]: flush forwarding table when device carrier off

Flush the forwarding table when carrier is lost. This helps for
availability because we don't want to forward to a downed device and
new packets may come in on other links.

Signed-off-by: Stephen Hemminger <shemminger@osdl.org>
Signed-off-by: David S. Miller <davem@davemloft.net>