netfilter: Fix potential use after free in ip6_route_me_harder()

Dst is released one line before we access it again with dst->error.

Fixes: 58e35d14 netfilter: ipv6: propagate routing errors from
ip6_route_me_harder()

Signed-off-by: Sergey Popovich <popovich_sergei@mail.ru>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>