[BRIDGE]: fix locking and memory leak in br_add_bridge

There are several bugs in error handling in br_add_bridge:
- when dev_alloc_name fails, allocated net_device is not freed
- unregister_netdev is called when rtnl lock is held
- free_netdev is called before netdev_run_todo has a chance to be run after
  unregistering net_device

Signed-off-by: Jiri Benc <jbenc@suse.cz>
Signed-off-by: Stephen Hemminger <shemminger@osdl.org>
Signed-off-by: David S. Miller <davem@davemloft.net>