bonding: Fix sysfs attribute handling

For bonding interfaces any attempt to read the sysfs directory contents after
module removal results in an oops.  The fix is to release sysfs attributes
for the interfaces upon module unload.

Signed-off-by: Libor Pechacek <lpechacek@suse.cz>
Acked-by: Jay Vosburgh <fubar@us.ibm.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>