bridge: Net device leak in br_add_bridge().

In case the register_netdevice() call fails the device is leaked,
since the out: label is just rtnl_unlock()+return.

Free the device.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>