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: NAlexey Dobriyan <adobriyan@gmail.com>
Acked-by: NStephen Hemminger <shemming@vyatta.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>