bonding: only use primary address for ARP

Only use the primary address of the bond device
for master_ip. This will prevent changing the ARP source
address in Active-Backup mode whenever a secondry address
is added to the bond device.

Signed-off-by: Henrik Saavedra Persson <henrik.e.persson@ericsson.com>
Signed-off-by: Andy Gospodarek <andy@greyhouse.net>
Signed-off-by: David S. Miller <davem@drr.davemloft.net>