selftests: forwarding: mirror_gre_bridge_1q_lag: Flush neighbors

After one LAG slave is downed and another upped, it takes a while for
the neighbor on a bridge to time out and get renegotiated. The test does
prompt update of FDB entries by arpinging. But because the neighbor
still references another address, offloading is not possible, and some
packets may end up not being mirrored.

To force the neighbor renegotiation, simply flush the neighbor table at
the bridge.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>