ofproto: Add CLI commands to show and clear mac_learning statistics
Add two new commands, fdb/stats-show and fdb/stats-clear, to
ovs-appctl to show and clear the new mac_learning statistics.
$ ovs-appctl fdb/stats-show ovs_pvp_br0
Statistics for bridge "ovs_pvp_br0":
Current/maximum MAC entries in the table: 4/2048
Total number of learned MAC entries : 4
Total number of expired MAC entries : 1
Total number of evicted MAC entries : 0
Total number of port moved MAC entries : 32