zebra: bridge layer2 information records ns_id where bridge is

zebra: bridge layer2 information records ns_id where bridge is

when working with vrf netns backend, two bridges interfaces may have the
same bridge interface index, but not the same namespace. because in vrf
netns backend mode, a bridge slave always belong to the same network
namespace, then a check with the namespace id and the ns id of the
bridge interface permits to resolve correctly the interface pointer.
The problem could occur if a same index of two bridge interfaces can be
found on two different namespaces.

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