bonding: use last_arp_rx in bond_loadbalance_arp_mon()

Now that last_arp_rx correctly show the last time we've received an ARP, we
can use it safely instead of slave->dev->last_rx.

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