bonding: always update last_arp_rx on packet recieve

Currently we're updating the last_arp_rx only when we've validate the
packet, however afterwards we use it as 'ANY last packet received', but not
only validated ARPs.

Fix this by updating it in case of any packet received. It won't break the
arp_validation=0 because we, anyway, return the correct slave->dev->last_rx in
slave_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>