netns bridge: allow bridges in netns!

Bridge as netdevice doesn't cross netns boundaries.

Bridge ports and bridge itself live in same netns.

Notifiers are fixed.

netns propagated from userspace socket for setup and teardown.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Acked-by: Stephen Hemminger <shemming@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>