netfilter: ipv6: fix afinfo->route refcnt leak on error

Several callers (h323 conntrack, xt_addrtype) assume that the
returned **dst only needs to be released if the function returns 0.

This is true for the ipv4 implementation, but not for the ipv6 one.

Instead of changing the users, change the ipv6 implementation
to behave like the ipv4 version by only providing the dst_entry result
in the success case.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>