netlink linux: enable listening to all nsids

netlink linux: enable listening to all nsids

Internal ports may be moved to another network namespace
and when that happens, the vswitch stops receiving netlink
notifications.

This patch enables the vswitch to listen to all network
namespaces that have a nsid assigned into the network
namespace where the socket has been opened.

It requires kernel 4.2 or newer.

Signed-off-by: Flavio Leitner <fbl@redhat.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>