bonding: fix bonding_masters race condition in bond unloading

While the bonding module is unloading, it is considered that after
rtnl_link_unregister all bond devices are destroyed but since no
synchronization mechanism exists, a new bond device can be created
via bonding_masters before unregister_pernet_subsys which would
lead to multiple problems (e.g. NULL pointer dereference, wrong RIP,
list corruption).

This patch fixes the issue by removing any bond devices left in the
netns after bonding_masters is removed from sysfs.

Signed-off-by: Nikolay Aleksandrov <nikolay@redhat.com>
Acked-by: Veaceslav Falico <vfalico@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>