Enhancement to MAC limit function

The handling of a burst of packets with new source MAC addresses is improved to reduce resource use and processing time. In earlier releases, new source MAC addresses are learned and placed in the MAC table even after the limit is exceeded. The Routing Engine later deletes the MAC address entries that are over the limit. Now, the learning limit configured with the interface-mac-limit statement for new source MAC addresses is enforced at all levels: global, bridge domain, and VPLS. The MAC table is not updated with any new addresses after the limit has been reached. When any static MAC addresses are configured, the learning limit is the configured limit minus the number of static addresses. When the configured packet action is drop, all subsequent packets with new source MAC addresses are dropped when the MAC address limit is reached. Otherwise, all such packets are forwarded when the MAC address limit is reached. This enhancement applies to the MAC address learning limit at all levels: global, bridge domain, and VPLS. It does not apply to bridge domain trunk ports, because those have no counters for the individual domains, which might have different MAC address learning limits. The enhancement also does not apply to aggregated Ethernet interfaces or to label-switched interfaces. In these cases, the behavior is to learn all the addresses and later delete the excess.