ipv6: Add hook to bump sernum for a route to stubs

Add hook to ipv6 stub to bump the sernum up to the root node for a
route. This is needed by the nexthop code when a nexthop config changes.

Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>