zebra: retrieve zns context from zvrf when netlink discovery

zebra: retrieve zns context from zvrf when netlink discovery

So as to get the correct NETNS where some discovery must be done and
populated, the zns pointer is directly retrieved from zvrf, instead of
checking that the VRF is a backend NETNS or not.
In the case where the interfaces are discovered before the VRF is enabled
( VRF-lite populate), then the default NS is retrieved.

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>