For the metrics other than MTU, we should also be getting it from
the top dst as opposed to the bottom dst.  With this change, the
user is able to manipulate values such as advmss for individual
hosts behind a remote IPsec gateway.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>