[INET]: Use port unreachable instead of proto for tunnels

This patch changes GRE and SIT to generate port unreachable instead of
protocol unreachable errors when we can't find a matching tunnel for a
packet.

This removes the ambiguity as to whether the error is caused by no
tunnel being found or by the lack of support for the given tunnel
type.

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