bridge: switch br_net_exit to batch mode

cleanup_net() is competing with other rtnl users.

Instead of calling br_net_exit() for each netns,
call br_net_exit_batch() once.

This gives cleanup_net() ability to group more devices
and call unregister_netdevice_many() only once for all bridge devices.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Roopa Prabhu <roopa@nvidia.com>
Cc: Nikolay Aleksandrov <razor@blackwall.org>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: David S. Miller <davem@davemloft.net>