zebra: accept static v6 routes with non-existent nexthop interfaces

zebra: accept static v6 routes with non-existent nexthop interfaces

We don't need to enforce that the interface exists because the route can
be activated later once the interface becomes available. We already do
this for IPv4 routes and IPv6 routes with both a nexthop address and a
nexthop interface.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>