bonding: always set recv_probe to bond_arp_rcv in arp monitor

Currently we only set bond_arp_rcv() if we're using arp_validate, however
this makes us skip updating last_arp_rx if we're not validating incoming
ARPs - thus, if arp_validate is off, last_arp_rx will never be updated.

Fix this by always setting up recv_probe = bond_arp_rcv, even if we're not
using arp_validate.

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>