bonding: change name of sysfs dir for bonding slaves

Allow user to identify easily what the attributes are related to. Change
the name of the group to "bonding_slave" to be similar to master which
is named "bonding".

Signed-off-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: David S. Miller <davem@davemloft.net>