zebra: avoid initialising more than one the l3vni hash structure

zebra: avoid initialising more than one the l3vni hash structure

the l3vni structure is allocated only once, since that structure is only
used for default netns. For that, move the initialisation part is moved
to a proper place, where there is no risk of attempting to initialise it
more than once, even when vrf backend is netns.

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