[IPSEC]: Set dst->input to dst_discard

The input function should never be invoked on IPsec dst objects.  This
is because we don't apply IPsec on input until after we've made the
routing decision.

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