net: Convert bond_net_ops

These pernet_operations populate/depopulate /proc and /sys
entries. Exit method unregisters all net bond devices, and
it seems another pernet_operations are not interested in
foreign net bond list. So, it's possible to mark them async.

Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: David S. Miller <davem@davemloft.net>