selftests: forwarding: mirror_gre_bridge_1q_lag: Ignore ARP

This test sets up mirroring such that it mirrors all overlay traffic.
That includes ARP, which causes occasional miscounts and spurious
failures. Ignore ARP explicitly to avoid these problems.

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