bonding: show full hw address in sysfs for slave entries

Bond expects ethernet hwaddr for its slave, but it can be longer than 6
bytes - infiniband interface for example.

 # cat /sys/devices/<skipped>/net/ib0/address
 80:00:02:08:fe:80:00:00:00:00:00:00:7c:fe:90:03:00:be:5d:e1

 # cat /sys/devices/<skipped>/net/ib0/bonding_slave/perm_hwaddr
 80:00:02:08:fe:80

So print full hwaddr in sysfs "bonding_slave/perm_hwaddr" as well.

Signed-off-by: Konstantin Khorenko <khorenko@virtuozzo.com>
Signed-off-by: David S. Miller <davem@davemloft.net>