batman-adv: use batadv_compare_eth for concise

It is better to use batadv_compate_eth instead of memcpy for
concise style.

Cc: Marek Lindner <mareklindner@neomailbox.ch>
Cc: Simon Wunderlich <sw@simonwunderlich.de>
Cc: Antonio Quartulli <antonio@meshcoding.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: b.a.t.m.a.n@lists.open-mesh.org
Cc: netdev@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Tan Xiaojun <tanxiaojun@huawei.com>
Signed-off-by: Ding Tianhong <dingtianhong@huawei.com>
Acked-by: Antonio Quartulli <antonio@meshcoding.com>
Signed-off-by: David S. Miller <davem@davemloft.net>