[INET]: Fix ECN encapsulation.

[INET]: Fix ECN encapsulation.

We broke ECN encapsulation in tunnels recently.
Without this patch, even though encapusulated (inner) packet is
'not-ECN', encapusulating (outer) packet is sent with 'ECT(0)' set.
This is wrong and should be 'not-ECN.'
This patch fixes up.

From RFC3168:
   The full-functionality option for ECN encapsulation is to copy the
   ECN codepoint of the inside header to the outside header on
   encapsulation if the inside header is not-ECT or ECT, and to set the
   ECN codepoint of the outside header to ECT(0) if the ECN codepoint of
   the inside header is CE.

Signed-off-by: Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>