netfilter: ipv6: unnecessary to check whether ip6_route_output() returns NULL

ip6_route_output() never returns NULL, so it is not appropriate to
check if the return value is NULL.

Signed-off-by: Haishuang Yan <yanhaishuang@cmss.chinamobile.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>