bonding: remove sysfs before removing devices

We have a race condition if we try to rmmod bonding and simultaneously add
a bond master through sysfs. In bonding_exit() we first remove the devices
(through rtnl_link_unregister() ) and only after that we remove the sysfs.
If we manage to add a device through sysfs after that the devices were
removed - we'll end up with that device/sysfs structure and with the module
unloaded.

Fix this by first removing the sysfs and only after that calling
rtnl_link_unregister().