bridge: fix use-after-free in br_cleanup_bridges()

Unregistering a bridge device may cause virtual devices stacked on the
bridge, like vlan or macvlan devices, to be unregistered as well.
br_cleanup_bridges() uses for_each_netdev_safe() to iterate over all
devices during cleanup. This is not enough however, if one of the
additionally unregistered devices is next in the list to the bridge
device, it will get freed as well and the iteration continues on
the freed element.

Restart iteration after each bridge device removal from the beginning to
fix this, similar to what rtnl_link_unregister() does.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Acked-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>