bonding: Remove net_device_stats from bonding struct

There is no need to maintain stats in the bonding structure.
Use the instance of net_device_stats in netdevice.

Signed-off-by: Ajit Khaparde <ajitk@serverengines.com>
Signed-off-by: Jay Vosburgh <fubar@us.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>