selftests: forwarding: tests of locked port feature

These tests check that the basic locked port feature works, so that
no 'host' can communicate (ping) through a locked port unless the
MAC address of the 'host' interface is in the forwarding database of
the bridge.

Signed-off-by: Hans Schultz <schultz.hans+netdev@gmail.com>
Acked-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>