zebra: map vxlan interface to bridge interface with correct ns id

zebra: map vxlan interface to bridge interface with correct ns id

an incoming bridge index has been found, that is linked with vxlan
interface, and the search for that bridge interface is done. In
vrf-lite, the search is done across the same default namespace, because
bridge and vxlan may not be in the same vrf. But this behaviour is wrong
when using vrf netns backend, as the bridge and the vxlan have to be in
the same vrf ( hence in the same network namespace). To comply with
that, use the netnamespace of the vxlan interface. Like that, the
appropriate nsid is passed as parameter, and consequently, the search is
correct, and the mac address passed to BGP will be ok too.

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