Currently it's disabled because it's sometimes hard, in typical configs, to
make it work - because of the nature how the loadbalance modes work - as
it's hard to deliver valid arp replies to correct slaves by the switch.

However we still can use arp_validation in loadbalance with several other
configs, per example with arp_validate == 2 for backup with one broadcast
domain, without the switch(es) doing any balancing - this way we'd be (a
bit more) sure that the slave is up.

So, enable it to let users decide which one works/suits them best. Also
correct the mode limitation from BOND_OPT_ARP_VALIDATE.

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