[IPV6]: Fix ICMP relookup error path dst leak

When we encounter an error while looking up the dst the second
time we need to drop the first dst.  This patch is pretty much
the same as the one for IPv4.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>