bonding: remove bond->lock from bond_arp_rcv

We're always called with rcu_read_lock() held (bond_arp_rcv() is only
called from bond_handle_frame(), which is rx_handler and always called
under rcu from __netif_receive_skb_core() ).

The slave active/passive and/or bonding params can change in-flight, however
we don't really care about that - we only modify the last time packet was
received, which is harmless.

CC: Jay Vosburgh <fubar@us.ibm.com>
CC: Andy Gospodarek <andy@greyhouse.net>
Signed-off-by: Veaceslav Falico <vfalico@redhat.com>
Acked-by: Ding Tianhong <dingtianhong@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>