netfilter: ipvs: fix dst leak in __ip_vs_addr_is_local_v6

After call to ip6_route_output() we must release dst or we leak it.

Also should test dst->error, as ip6_route_output() never returns NULL.

Use boolean while we are at it.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>