bridge: fix a possible net_device leak

Jan Beulich reported a possible net_device leak in bridge code after
commit bb900b27

 (bridge: allow creating bridge devices with netlink)

Reported-by: Jan Beulich <JBeulich@novell.com>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Acked-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>